[DevTools] Turn starting tracing into promise.

third_party/WebKit/Source/devtools/front_end/sdk/TracingManager.js

 /*
  * Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 /**
  * @interface
  */
 SDK.TracingManagerClient = function() {};
 
 SDK.TracingManagerClient.prototype = {
-  tracingStarted() {},
   /**
    * @param {!Array.<!SDK.TracingManager.EventPayload>} events
    */
   traceEventsCollected(events) {},
   tracingComplete() {},
   /**
    * @param {number} usage
    */
   tracingBufferUsage(usage) {},
   /**
@@ skipping 54 matching lines @@
     this._eventsRetrieved = 0;
     this._activeClient.tracingComplete();
     this._activeClient = null;
     this._finishing = false;
   }
 
   /**
    * @param {!SDK.TracingManagerClient} client
    * @param {string} categoryFilter
    * @param {string} options
-   * @param {function(?string)=} callback
+   * @return {!Promise}
    */
-  start(client, categoryFilter, options, callback) {
+  start(client, categoryFilter, options) {
     if (this._activeClient)
       throw new Error('Tracing is already started');
     var bufferUsageReportingIntervalMs = 500;
     this._activeClient = client;
-    this._target.tracingAgent().start(
-        categoryFilter, options, bufferUsageReportingIntervalMs, SDK.TracingManager.TransferMode.ReportEvents,
-        callback);
-    this._activeClient.tracingStarted();
+    return this._target.tracingAgent().start(
+        categoryFilter, options, bufferUsageReportingIntervalMs, SDK.TracingManager.TransferMode.ReportEvents);
   }
 
   stop() {
     if (!this._activeClient)
       throw new Error('Tracing is not started');
     if (this._finishing)
       throw new Error('Tracing is already being stopped');
     this._finishing = true;
     this._target.tracingAgent().end();
   }
@@ skipping 52 matching lines @@
     this._tracingManager._eventsCollected(data);
   }
 
   /**
    * @override
    */
   tracingComplete() {
     this._tracingManager._tracingComplete();
   }
 };