ui::Layer::SwitchToLayer should finish animations on its old cc::Layer

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index 60057ca0762bd33feee9495b1dd1be6417d61364..6ad646314050054be4aaed91961eea76031d0660 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -316,6 +316,9 @@ class COMPOSITOR_EXPORT Layer
     return pending_threaded_animations_.size() != 0;
   }
 
+  // Triggers a call to SwitchToLayer.
+  void SwitchCCLayerForTest();
+
  private:
   // Stacks |child| above or below |other|.  Helper method for StackAbove() and
   // StackBelow().