Refactor MutatorHostClient from LayerTreeHost to LayerTree.

cc/trees/tree_synchronizer.cc

Index: cc/trees/tree_synchronizer.cc
diff --git a/cc/trees/tree_synchronizer.cc b/cc/trees/tree_synchronizer.cc
index 6314afbae0ab754c5d26f1a837ff9eb789c6935e..30535213c6fdbc39a4df3afee7d477431c42168a 100644
--- a/cc/trees/tree_synchronizer.cc
+++ b/cc/trees/tree_synchronizer.cc
@@ -46,7 +46,7 @@ void TreeSynchronizer::SynchronizeTrees(Layer* layer_root,
   if (!layer_root) {
     tree_impl->DetachLayers();
   } else {
-    SynchronizeTreesInternal(layer_root->layer_tree_host(), tree_impl,
+    SynchronizeTreesInternal(layer_root->GetLayerTree(), tree_impl,
                              layer_root->layer_tree_host()->property_trees());
   }
 }