feat(Page): introduce workers (#2560)

This adds `page.workers()`, and two events `workercreated` and `workerdestroyed`. It also forwards logs from a worker into the page `console` event.

Only dedicated workers are supported for now, ServiceWorkers will probably work differently because they aren't necessarily associated with a single page.

Fixes #2350.

lib/Worker.js

@@ -0,0 +1,59 @@
+/**
+ * Copyright 2018 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 EventEmitter = require('events');
+const {helper, debugError} = require('./helper');
+const {ExecutionContext, JSHandle} = require('./ExecutionContext');
+
+class Worker extends EventEmitter {
+  /**
+   * @param {Puppeteer.CDPSession} client
+   * @param {string} url
+   * @param {function(!Protocol.Log.entryAddedPayload)} logEntryAdded
+   */
+  constructor(client, url, logEntryAdded) {
+    super();
+    this._client = client;
+    this._url = url;
+    this._executionContextPromise = new Promise(x => this._executionContextCallback = x);
+    this._client.on('Runtime.executionContextCreated', event => {
+      const jsHandleFactory = remoteObject => new JSHandle(executionContext, client, remoteObject);
+      const executionContext = new ExecutionContext(client, event.context, jsHandleFactory, null);
+      this._executionContextCallback(executionContext);
+    });
+    // This might fail if the target is closed before we recieve all execution contexts.
+    this._client.send('Runtime.enable', {}).catch(debugError);
+
+    this._client.on('Log.entryAdded', logEntryAdded);
+    this._client.send('Log.enable', {}).catch(debugError);
+  }
+
+  /**
+   * @return {string}
+   */
+  url() {
+    return this._url;
+  }
+
+  /**
+   * @return {!Promise<ExecutionContext>}
+   */
+  async executionContext() {
+    return this._executionContextPromise;
+  }
+}
+
+module.exports = Worker;
+helper.tracePublicAPI(Worker);