DevTools: properly await profiler start before starting tracing

we were waiting for undefined instead of a promise.

Bug: 775756
Change-Id: I2fcddae0df9aaab48aa803787b46e87aa72ed61e
Reviewed-on: https://chromium-review.googlesource.com/724427Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509673}

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

@@ -113,11 +113,14 @@ SDK.CPUProfilerModel = class extends SDK.SDKModel {
     return this._isRecording;
   }
 
+  /**
+   * @return {!Promise}
+   */
   startRecording() {
     this._isRecording = true;
     var intervalUs = Common.moduleSetting('highResolutionCpuProfiling').get() ? 100 : 1000;
     this._profilerAgent.setSamplingInterval(intervalUs);
-    this._profilerAgent.start();
+    return this._profilerAgent.start();
   }
 
   /**