Implement browser.stdout and browser.stderr streams

This patch adds browser.stdout and browser.stderr streams.

These streams allow to get the browser instance output, e.g.

```js
browser.stderr.pipe(process.stdout);
```

lib/Browser.js

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+let {Duplex} = require('stream');
 let http = require('http');
 let path = require('path');
 let removeRecursive = require('rimraf').sync;
@@ -64,6 +65,9 @@ class Browser {
     this._terminated = false;
     this._chromeProcess = null;
     this._launchPromise = null;
+
+    this.stderr = new ProxyStream();
+    this.stdout = new ProxyStream();
   }
 
   /**
@@ -112,6 +116,8 @@ class Browser {
       this._terminated = true;
       removeRecursive(this._userDataDir);
     });
+    this._chromeProcess.stderr.pipe(this.stderr);
+    this._chromeProcess.stdout.pipe(this.stdout);
 
     await waitForChromeResponsive(this._remoteDebuggingPort, () => !this._terminated);
     if (this._terminated)
@@ -153,3 +159,17 @@ function waitForChromeResponsive(remoteDebuggingPort, shouldWaitCallback) {
     req.end();
   }
 }
+
+class ProxyStream extends Duplex {
+  _read() { }
+
+  /**
+   * @param {?} chunk
+   * @param {string} encoding
+   * @param {function()} callback
+   */
+  _write(chunk, encoding, callback) {
+    this.push(chunk, encoding);
+    callback();
+  }
+}