CC Animations: Make LayerAnimationController creation optional

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 14ceca4f9085784552b7062b3dc560ec8ccfcd8a..7d643b9d41fb02213829602b5167bd48c5797a15 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -400,6 +400,11 @@ const cc::RendererSettings& Compositor::GetRendererSettings() const {
   return host_->settings().renderer_settings;
 }
 
+const cc::LayerTreeHost& Compositor::GetLayerTreeHost() const {
+  DCHECK(host_);
+  return *host_.get();
+}
+
 scoped_refptr<CompositorLock> Compositor::GetCompositorLock() {
   if (!compositor_lock_) {
     compositor_lock_ = new CompositorLock(this);