[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 5ed718cfe578327fe79bff22fb1791749cf9f7ea..f30507f8ddc10b7e1614720e18d59b226fa42db8 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -545,6 +545,10 @@ const cc::RendererSettings& Compositor::GetRendererSettings() const {
   return host_->GetSettings().renderer_settings;
 }
 
+int Compositor::GetFrameNumber() {
+  return host_->SourceFrameNumber();

[ajuma, 2017/01/18 23:02:51]

This number won't get incremented for frames that are impl-side only in cc. Even
though we don't have a compositor thread for ui, we're still able to tick
"composited" animations without going through the "main-thread" side code, so if
the animation is only for transform or opacity the SourceFrameNumber might only
get incremented at the beginning and end.

Instead, adding a count to calls to Compositor::DidSubmitCompositorFrame (either
directly or by adding a CompositorObserver) would work.

[varkha, 2017/01/20 00:16:47]

Done.

+}
+
 scoped_refptr<CompositorLock> Compositor::GetCompositorLock() {
   if (!compositor_lock_) {
     compositor_lock_ = new CompositorLock(this);