[DevTools] Turn starting tracing into promise.

third_party/WebKit/Source/devtools/front_end/Tests.js

Index: third_party/WebKit/Source/devtools/front_end/Tests.js
diff --git a/third_party/WebKit/Source/devtools/front_end/Tests.js b/third_party/WebKit/Source/devtools/front_end/Tests.js
index 11a6be78ff7b12d82bd3250d9956e2432a2bff81..8a2af93c771199eeb03257597bf8c306f7785316 100644
--- a/third_party/WebKit/Source/devtools/front_end/Tests.js
+++ b/third_party/WebKit/Source/devtools/front_end/Tests.js
@@ -912,7 +912,7 @@
     var test = this;
     this.showPanel('timeline').then(function() {
       var timeline = UI.panels.timeline;
-      test._overrideMethod(timeline, 'recordingStarted', callback);
+      test._overrideMethod(timeline, '_recordingStarted', callback);
       timeline._toggleRecording();
     });
   };