feat(launcher): add experimental "transport" option to pptr.connect (#3265)

This patch:
- adds experimental "transport" option to pptr.connect
- uses "transport" option to make sure Puppeteer-Web works with
  Target.exposeDevToolsProtocol

Drive-by: add `browser.target()` to access browser target.

lib/Browser.js

@@ -185,6 +185,13 @@ class Browser extends EventEmitter {
     return Array.from(this._targets.values()).filter(target => target._isInitialized);
   }
 
+  /**
+   * @return {!Target}
+   */
+  target() {
+    return this.targets().find(target => target.type() === 'browser');
+  }
+
   /**
    * @return {!Promise<!Array<!Puppeteer.Page>>}
    */

lib/Launcher.js

@@ -270,7 +270,7 @@ class Launcher {
   }
 
   /**
-   * @param {!(BrowserOptions & {browserWSEndpoint: string})} options
+   * @param {!(BrowserOptions & {browserWSEndpoint: string, transport?: !Puppeteer.ConnectionTransport})} options
    * @return {!Promise<!Browser>}
    */
   async connect(options) {
@@ -278,9 +278,9 @@ class Launcher {
       browserWSEndpoint,
       ignoreHTTPSErrors = false,
       defaultViewport = {width: 800, height: 600},
+      transport = await WebSocketTransport.create(browserWSEndpoint),
       slowMo = 0,
     } = options;
-    const transport = await WebSocketTransport.create(browserWSEndpoint);
     const connection = new Connection(browserWSEndpoint, transport, slowMo);
     const {browserContextIds} = await connection.send('Target.getBrowserContexts');
     return Browser.create(connection, browserContextIds, ignoreHTTPSErrors, defaultViewport, null, () => connection.send('Browser.close').catch(debugError));

lib/Puppeteer.js

@@ -37,7 +37,7 @@ module.exports = class {
   }
 
   /**
-   * @param {{browserWSEndpoint: string, ignoreHTTPSErrors: boolean}} options
+   * @param {{browserWSEndpoint: string, ignoreHTTPSErrors: boolean, transport?: !Puppeteer.ConnectionTransport}} options
    * @return {!Promise<!Puppeteer.Browser>}
    */
   connect(options) {