Add BeginFrameLatency UMA

cc/scheduler/compositor_timing_history.cc

Index: cc/scheduler/compositor_timing_history.cc
diff --git a/cc/scheduler/compositor_timing_history.cc b/cc/scheduler/compositor_timing_history.cc
index b1a9837020df43b9c70e09107d9d05dd68e5c1e0..704380d30af999a33c60aab3ca6378cfc38d8e98 100644
--- a/cc/scheduler/compositor_timing_history.cc
+++ b/cc/scheduler/compositor_timing_history.cc
@@ -26,6 +26,7 @@ class CompositorTimingHistory::UMAReporter {
   virtual void AddDrawInterval(base::TimeDelta interval) = 0;
 
   // Latency measurements
+  virtual void AddBeginMainFrameLatency(base::TimeDelta delta) = 0;
   virtual void AddBeginMainFrameQueueDurationCriticalDuration(
       base::TimeDelta duration) = 0;
   virtual void AddBeginMainFrameQueueDurationNotCriticalDuration(
@@ -197,6 +198,11 @@ class RendererUMAReporter : public CompositorTimingHistory::UMAReporter {
     UMA_HISTOGRAM_CUSTOM_TIMES_VSYNC_ALIGNED(
         "Scheduling.Renderer.MainAndImplFrameTimeDelta", delta);
   }
+
+  void AddBeginMainFrameLatency(base::TimeDelta delta) override {
+    UMA_HISTOGRAM_CUSTOM_TIMES_DURATION(
+        "Scheduling.Renderer.VsyncToBeginMainFrameLatency", delta);
+  }
 };
 
 class BrowserUMAReporter : public CompositorTimingHistory::UMAReporter {
@@ -274,6 +280,11 @@ class BrowserUMAReporter : public CompositorTimingHistory::UMAReporter {
     UMA_HISTOGRAM_CUSTOM_TIMES_VSYNC_ALIGNED(
         "Scheduling.Browser.MainAndImplFrameTimeDelta", delta);
   }
+
+  void AddBeginMainFrameLatency(base::TimeDelta delta) override {
+    UMA_HISTOGRAM_CUSTOM_TIMES_DURATION(
+        "Scheduling.Browser.VsyncToBeginMainFrameLatency", delta);
+  }
 };
 
 class NullUMAReporter : public CompositorTimingHistory::UMAReporter {
@@ -297,6 +308,7 @@ class NullUMAReporter : public CompositorTimingHistory::UMAReporter {
   void AddSubmitToAckLatency(base::TimeDelta duration) override {}
   void AddSubmitAckWasFast(bool was_fast) override {}
   void AddMainAndImplFrameTimeDelta(base::TimeDelta delta) override {}
+  void AddBeginMainFrameLatency(base::TimeDelta delta) override {}
 };
 
 }  // namespace
@@ -500,6 +512,9 @@ void CompositorTimingHistory::WillBeginMainFrame(
   begin_main_frame_sent_time_ = Now();
   begin_main_frame_frame_time_ = main_frame_time;
 
+  uma_reporter_->AddBeginMainFrameLatency(begin_main_frame_sent_time_ -
+                                          begin_main_frame_frame_time_);
+
   did_send_begin_main_frame_ = true;
   SetBeginMainFrameNeededContinuously(true);
 }