Add latency measurement system

ui/compositor/compositor_observer.h

Index: ui/compositor/compositor_observer.h
diff --git a/ui/compositor/compositor_observer.h b/ui/compositor/compositor_observer.h
index 4157453ec6004d0224dfce198f04a635e2921c7b..2f0056ee4beeb82ac20b0ed15c9002543413537f 100644
--- a/ui/compositor/compositor_observer.h
+++ b/ui/compositor/compositor_observer.h
@@ -7,6 +7,10 @@
 
 #include "ui/compositor/compositor_export.h"
 
+namespace cc {
+struct LatencyInfo;
+}
+
 namespace ui {
 
 class Compositor;
@@ -36,6 +40,11 @@ class COMPOSITOR_EXPORT CompositorObserver {
   // Called when the compositor lock state changes.
   virtual void OnCompositingLockStateChanged(Compositor* compositor) = 0;
 
+  // Called when a frame has been displayed and latency info for it was
+  // received.
+  virtual void OnReceivedLatencyInfo(Compositor* compositor,
+      const cc::LatencyInfo& latency_info) = 0;
+
  protected:
   virtual ~CompositorObserver() {}
 };