[DevTools] Turn starting tracing into promise.

third_party/WebKit/LayoutTests/inspector/tracing/buffer-usage.html

Index: third_party/WebKit/LayoutTests/inspector/tracing/buffer-usage.html
diff --git a/third_party/WebKit/LayoutTests/inspector/tracing/buffer-usage.html b/third_party/WebKit/LayoutTests/inspector/tracing/buffer-usage.html
index cc6e1d2cbf7aa120a974dbd4d021144580bb6e25..ff7b9c2f1537261e1e89fc0471ff4b7fec341423 100644
--- a/third_party/WebKit/LayoutTests/inspector/tracing/buffer-usage.html
+++ b/third_party/WebKit/LayoutTests/inspector/tracing/buffer-usage.html
@@ -12,11 +12,6 @@ function test()
     }
 
     TestTimelineControllerClient.prototype = {
-        recordingStarted: function()
-        {
-            InspectorTest.addResult("TimelineControllerClient.recordingStarted");
-        },
-
         recordingProgress: function()
         {
             if (!controller)
@@ -52,7 +47,9 @@ function test()
     };
     var performanceModel = new Timeline.PerformanceModel();
     var controller = new Timeline.TimelineController(SDK.targetManager.mainTarget(), performanceModel, new TestTimelineControllerClient());
-    controller.startRecording({}, []);
+    controller.startRecording({}, []).then(() => {
+        InspectorTest.addResult("TimelineControllerClient.recordingStarted");
+    });
 }
 
 </script>