Use puppeteer.launch instead of browser constructor (#255)

This patch:
- split browser launching logic from Browser into `lib/Launcher.js`
- introduce `puppeteer` namespace which currently has a single `launch`
  method to start a browser

With this patch, the browser is no longer created with the `new
Browser(..)` command. Instead, it should be "launched" via the
`puppeteer.launch` method:

```js
const puppeteer = require('puppeteer');
puppeteer.launch().then(async browser => {
  ...
});
```

With this approach browser instance lifetime matches the lifetime of
actual browser process. This helps us:
- remove proxy streams, e.g. browser.stderr and browser.stdout
- cleanup browser class and make it possible to connect to remote
  browser
- introduce events on the browser instance, e.g. 'page' event. In case
  of lazy-launching browser, we should've launch browser when an event
  listener is added, which is unneded comlpexity.

lib/Launcher.js

@@ -0,0 +1,128 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+const path = require('path');
+const removeRecursive = require('rimraf').sync;
+const childProcess = require('child_process');
+const Downloader = require('../utils/ChromiumDownloader');
+const Connection = require('./Connection');
+const Browser = require('./Browser');
+const readline = require('readline');
+
+const CHROME_PROFILE_PATH = path.resolve(__dirname, '..', '.dev_profile');
+let browserId = 0;
+
+const DEFAULT_ARGS = [
+  '--disable-background-networking',
+  '--disable-background-timer-throttling',
+  '--disable-client-side-phishing-detection',
+  '--disable-default-apps',
+  '--disable-hang-monitor',
+  '--disable-popup-blocking',
+  '--disable-prompt-on-repost',
+  '--disable-sync',
+  '--enable-automation',
+  '--metrics-recording-only',
+  '--no-first-run',
+  '--password-store=basic',
+  '--remote-debugging-port=0',
+  '--safebrowsing-disable-auto-update',
+  '--use-mock-keychain',
+];
+
+class Launcher {
+  /**
+   * @param {!Object} options
+   * @return {!Promise<!Browser>}
+   */
+  static async launch(options) {
+    options = options || {};
+    ++browserId;
+    let userDataDir = CHROME_PROFILE_PATH + browserId;
+    let chromeArguments = DEFAULT_ARGS.concat([
+      `--user-data-dir=${userDataDir}`,
+    ]);
+    if (typeof options.headless !== 'boolean' || options.headless) {
+      chromeArguments.push(
+          `--headless`,
+          `--disable-gpu`,
+          `--hide-scrollbars`
+      );
+    }
+    let chromeExecutable = options.executablePath;
+    if (typeof chromeExecutable !== 'string') {
+      let chromiumRevision = require('../package.json').puppeteer.chromium_revision;
+      let revisionInfo = Downloader.revisionInfo(Downloader.currentPlatform(), chromiumRevision);
+      console.assert(revisionInfo, 'Chromium revision is not downloaded. Run npm install');
+      chromeExecutable = revisionInfo.executablePath;
+    }
+    if (Array.isArray(options.args))
+      chromeArguments.push(...options.args);
+    let chromeProcess = childProcess.spawn(chromeExecutable, chromeArguments, {});
+    if (options.dumpio) {
+      chromeProcess.stdout.pipe(process.stdout);
+      chromeProcess.stderr.pipe(process.stderr);
+    }
+    let stderr = '';
+    chromeProcess.stderr.on('data', data => stderr += data.toString('utf8'));
+    // Cleanup as processes exit.
+    const onProcessExit = () => chromeProcess.kill();
+    process.on('exit', onProcessExit);
+    let terminated = false;
+    chromeProcess.on('exit', () => {
+      terminated = true;
+      process.removeListener('exit', onProcessExit);
+      removeRecursive(userDataDir);
+    });
+
+    let {port, browserTargetId} = await waitForRemoteDebuggingPort(chromeProcess);
+    if (terminated)
+      throw new Error('Failed to launch chrome! ' + stderr);
+    // Failed to connect to browser.
+    if (port === -1) {
+      chromeProcess.kill();
+      throw new Error('Failed to connect to chrome!');
+    }
+
+    let connectionDelay = options.slowMo || 0;
+    let connection = await Connection.create(port, browserTargetId, connectionDelay);
+    return new Browser(connection, !!options.ignoreHTTPSErrors, () => chromeProcess.kill());
+  }
+}
+
+/**
+ * @param {!ChildProcess} chromeProcess
+ * @return {!Promise<number>}
+ */
+function waitForRemoteDebuggingPort(chromeProcess) {
+  return new Promise(fulfill => {
+    const rl = readline.createInterface({ input: chromeProcess.stderr });
+    rl.on('line', onLine);
+    rl.once('close', () => fulfill(-1));
+
+    /**
+     * @param {string} line
+     */
+    function onLine(line) {
+      const match = line.match(/^DevTools listening on .*:(\d+)(\/.*)$/);
+      if (!match)
+        return;
+      rl.removeListener('line', onLine);
+      fulfill({port: Number.parseInt(match[1], 10), browserTargetId: match[2]});
+    }
+  });
+}
+
+module.exports = Launcher;