Refactor MutatorHostClient from LayerTreeHost to LayerTree.

cc/animation/animation_host_perftest.cc

Index: cc/animation/animation_host_perftest.cc
diff --git a/cc/animation/animation_host_perftest.cc b/cc/animation/animation_host_perftest.cc
index 8d855e5d4d66b76423b92b3fdc4bafccc82cca77..3d43ff69450bb3597a3af5908a4f3236893c535a 100644
--- a/cc/animation/animation_host_perftest.cc
+++ b/cc/animation/animation_host_perftest.cc
@@ -44,7 +44,9 @@ class AnimationHostPerfTest : public testing::Test {
     layer_tree_host_ = nullptr;
   }
 
-  AnimationHost* host() const { return layer_tree_host_->animation_host(); }
+  AnimationHost* host() const {
+    return layer_tree_host_->GetLayerTree()->animation_host();
+  }
   AnimationHost* host_impl() const {
     return layer_tree_host_->host_impl()->animation_host();
   }