CC Animation: Let LayerTreeHost and LayerTree use MutatorHost interface.

cc should work with cc/animation via MutatorHost abstract interface.

This is a continuation to this CL:
https://codereview.chromium.org/2024393002

[email protected]
BUG=575053
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2476893002
Cr-Commit-Position: refs/heads/master@{#430540}
diff --git a/content/renderer/gpu/render_widget_compositor.h b/content/renderer/gpu/render_widget_compositor.h
index 02e3235..e51ec83 100644
--- a/content/renderer/gpu/render_widget_compositor.h
+++ b/content/renderer/gpu/render_widget_compositor.h
@@ -29,6 +29,8 @@
 }
 
 namespace cc {
+
+class AnimationHost;
 class InputHandler;
 class Layer;
 class LayerTreeHost;
@@ -223,6 +225,7 @@
   RenderWidgetCompositorDelegate* const delegate_;
   CompositorDependencies* const compositor_deps_;
   const bool threaded_;
+  std::unique_ptr<cc::AnimationHost> animation_host_;
   std::unique_ptr<cc::LayerTreeHost> layer_tree_host_;
   bool never_visible_;