Refactor MutatorHostClient from LayerTreeHost to LayerTree.

cc/test/layer_tree_test.cc

Index: cc/test/layer_tree_test.cc
diff --git a/cc/test/layer_tree_test.cc b/cc/test/layer_tree_test.cc
index 86bb2e0a75efd304abdbd557b76a664b34f1bfb9..7935af5c20a11389e458b9ea44b2ecaba4c818b3 100644
--- a/cc/test/layer_tree_test.cc
+++ b/cc/test/layer_tree_test.cc
@@ -930,6 +930,10 @@ TaskRunnerProvider* LayerTreeTest::task_runner_provider() const {
   return host->task_runner_provider();
 }
 
+LayerTree* LayerTreeTest::layer_tree() {
+  return layer_tree_host()->GetLayerTree();
+}
+
 LayerTreeHost* LayerTreeTest::layer_tree_host() {
   DCHECK(task_runner_provider()->IsMainThread() ||
          task_runner_provider()->IsMainThreadBlocked());