[exp] cc: Introduce cc::CompositorMutator.

cc/layers/picture_layer_unittest.cc

Index: cc/layers/picture_layer_unittest.cc
diff --git a/cc/layers/picture_layer_unittest.cc b/cc/layers/picture_layer_unittest.cc
index ab06bb01b81d9ffa09fd8237228043bf61e11b9d..be3d5ab30e2b22dc047a77b542b182d0fd2262ec 100644
--- a/cc/layers/picture_layer_unittest.cc
+++ b/cc/layers/picture_layer_unittest.cc
@@ -130,12 +130,12 @@ TEST(PictureLayerTest, NonMonotonicSourceFrameNumber) {
 
   scoped_ptr<LayerTreeHost> host1 = LayerTreeHost::CreateSingleThreaded(
       &host_client1, &host_client1, shared_bitmap_manager.get(), nullptr,
-      settings, base::MessageLoopProxy::current(), nullptr);
+      nullptr, settings, base::MessageLoopProxy::current(), nullptr);
   host_client1.SetLayerTreeHost(host1.get());
 
   scoped_ptr<LayerTreeHost> host2 = LayerTreeHost::CreateSingleThreaded(
       &host_client2, &host_client2, shared_bitmap_manager.get(), nullptr,
-      settings, base::MessageLoopProxy::current(), nullptr);
+      nullptr, settings, base::MessageLoopProxy::current(), nullptr);
   host_client2.SetLayerTreeHost(host2.get());
 
   // The PictureLayer is put in one LayerTreeHost.