[exp] cc: Introduce cc::CompositorMutator.

cc/layers/layer_unittest.cc

Index: cc/layers/layer_unittest.cc
diff --git a/cc/layers/layer_unittest.cc b/cc/layers/layer_unittest.cc
index 8a615100378379a2a542c57aa989758ff558bcbb..9f5315900485b9dec01a786558fb93f394625f2f 100644
--- a/cc/layers/layer_unittest.cc
+++ b/cc/layers/layer_unittest.cc
@@ -41,7 +41,7 @@ namespace {
 class MockLayerTreeHost : public LayerTreeHost {
  public:
   explicit MockLayerTreeHost(FakeLayerTreeHostClient* client)
-      : LayerTreeHost(client, nullptr, nullptr, LayerTreeSettings()) {
+      : LayerTreeHost(client, nullptr, nullptr, nullptr, LayerTreeSettings()) {
     InitializeSingleThreaded(client,
                              base::MessageLoopProxy::current(),
                              nullptr);
@@ -935,24 +935,16 @@ class LayerTreeHostFactory {
 
   scoped_ptr<LayerTreeHost> Create() {
     return LayerTreeHost::CreateSingleThreaded(
-               &client_,
-               &client_,
-               shared_bitmap_manager_.get(),
-               gpu_memory_buffer_manager_.get(),
-               LayerTreeSettings(),
-               base::MessageLoopProxy::current(),
-               nullptr);
+        &client_, &client_, shared_bitmap_manager_.get(),
+        gpu_memory_buffer_manager_.get(), nullptr, LayerTreeSettings(),
+        base::MessageLoopProxy::current(), nullptr);
   }
 
   scoped_ptr<LayerTreeHost> Create(LayerTreeSettings settings) {
     return LayerTreeHost::CreateSingleThreaded(
-               &client_,
-               &client_,
-               shared_bitmap_manager_.get(),
-               gpu_memory_buffer_manager_.get(),
-               settings,
-               base::MessageLoopProxy::current(),
-               nullptr);
+        &client_, &client_, shared_bitmap_manager_.get(),
+        gpu_memory_buffer_manager_.get(), nullptr, settings,
+        base::MessageLoopProxy::current(), nullptr);
   }
 
  private: