Refactor MutatorHostClient from LayerTreeHost to LayerTree.

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index ce6ea32b5c014b0713a6c4e495e023a85c361186..ba578dfd67e23299810e8012b6aff1840835a46e 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -690,15 +690,18 @@ void RenderWidgetCompositor::clearRootLayer() {
 
 void RenderWidgetCompositor::attachCompositorAnimationTimeline(
     cc::AnimationTimeline* compositor_timeline) {
-  DCHECK(layer_tree_host_->animation_host());
-  layer_tree_host_->animation_host()->AddAnimationTimeline(compositor_timeline);
+  cc::AnimationHost* animation_host = layer_tree_host_->GetLayerTree()
+      ->animation_host();
+  DCHECK(animation_host);
+  animation_host->AddAnimationTimeline(compositor_timeline);
 }
 
 void RenderWidgetCompositor::detachCompositorAnimationTimeline(
     cc::AnimationTimeline* compositor_timeline) {
-  DCHECK(layer_tree_host_->animation_host());
-  layer_tree_host_->animation_host()->RemoveAnimationTimeline(
-      compositor_timeline);
+  cc::AnimationHost* animation_host = layer_tree_host_->GetLayerTree()
+      ->animation_host();
+  DCHECK(animation_host);
+  animation_host->RemoveAnimationTimeline(compositor_timeline);
 }
 
 void RenderWidgetCompositor::setViewportSize(