[animations] Adds metrics for jank on selected layer animations

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index fb0fe72e4786b2f078102bc32582b471bf288cef..b39ea25fff877910e6d15e64ccd3281ad5965e12 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -80,6 +80,7 @@ Compositor::Compositor(const cc::FrameSinkId& frame_sink_id,
       context_factory_private_(context_factory_private),
       root_layer_(NULL),
       widget_(gfx::kNullAcceleratedWidget),
+      committed_frame_number_(0),
       widget_valid_(false),
       compositor_frame_sink_requested_(false),
       frame_sink_id_(frame_sink_id),
@@ -506,6 +507,7 @@ void Compositor::DidCommit() {
 }
 
 void Compositor::DidReceiveCompositorFrameAck() {
+  ++committed_frame_number_;
   for (auto& observer : observer_list_)
     observer.OnCompositingEnded(this);
 }