[animations] Adds metrics for jank on selected layer animations

ui/compositor/compositor.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/compositor/compositor.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <deque>
@@ skipping 527 matching lines @@
 
 void Compositor::SetLayerTreeDebugState(
     const cc::LayerTreeDebugState& debug_state) {
   host_->SetDebugState(debug_state);
 }
 
 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);
     host_->SetDeferCommits(true);
     for (auto& observer : observer_list_)
       observer.OnCompositingLockStateChanged(this);
   }
   return compositor_lock_;
 }
 
 void Compositor::UnlockCompositor() {
   DCHECK(compositor_lock_);
   compositor_lock_ = NULL;
   host_->SetDeferCommits(false);
   for (auto& observer : observer_list_)
     observer.OnCompositingLockStateChanged(this);
 }
 
 void Compositor::CancelCompositorLock() {
   if (compositor_lock_)
     compositor_lock_->CancelLock();
 }
 
 }  // namespace ui