Add trace event for detecting main frame in loading metrics.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 632baad692802bcfb2f4633adcbb69d9286325cd..b7bc552fa37ce56364e57c298b8422407406d5f3 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -940,6 +940,9 @@ RenderFrameImpl* RenderFrameImpl::CreateMainFrame(
   DCHECK_EQ(render_view->GetWidget(), render_frame->render_widget_)
       << "Main frame is no longer reusing the RenderView as its widget! "
       << "Does the RenderFrame need to register itself with the RenderWidget?";
+  TRACE_EVENT_INSTANT1("loading", "createMainFrame", TRACE_EVENT_SCOPE_THREAD,
+                       "frame", web_frame->frameIdForTracing());
+
   return render_frame;
 }