Record active tree main frame and impl frame times for delta metric.

To help debug the Scheduling.Browser.MainAndImplFrameTimeDelta2 metric, this
adds a trace of the times used to compute the impl and main frame delta. We
should be able to use this to identify which frames were identified as slipping
and when those times were recorded.

BUG=719667

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I6ab1d6ca72f0c3ec186740d33014e8d39cfe8404
Reviewed-on: https://chromium-review.googlesource.com/535658Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Brian Anderson <brianderson@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#481928}

cc/scheduler/compositor_timing_history.cc

@@ -714,6 +714,10 @@ void CompositorTimingHistory::DidDraw(bool used_new_active_tree,
     base::TimeDelta main_and_impl_delta =
         impl_frame_time - active_tree_main_frame_time_;
     DCHECK_GE(main_and_impl_delta, base::TimeDelta());
+    TRACE_EVENT2(TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"),
+                 "CompositorTimingHistory::DidDraw",
+                 "active_tree_main_frame_time", active_tree_main_frame_time_,
+                 "impl_frame_time", impl_frame_time);
     uma_reporter_->AddMainAndImplFrameTimeDelta(main_and_impl_delta);
     active_tree_main_frame_time_ = base::TimeTicks();