[exp] cc: Introduce cc::CompositorMutator.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index a8d4294bb418d6754becc31a19cb68ea73682e22..a623b414f9a62f8081ddcde0d76bfab45a8b8a55 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -168,14 +168,12 @@ scoped_ptr<LayerTreeHostImpl> LayerTreeHostImpl::Create(
     RenderingStatsInstrumentation* rendering_stats_instrumentation,
     SharedBitmapManager* shared_bitmap_manager,
     gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager,
+    CompositorMutator* compositor_mutator,
     int id) {
-  return make_scoped_ptr(new LayerTreeHostImpl(settings,
-                                               client,
-                                               proxy,
-                                               rendering_stats_instrumentation,
-                                               shared_bitmap_manager,
-                                               gpu_memory_buffer_manager,
-                                               id));
+  return make_scoped_ptr(new LayerTreeHostImpl(
+      settings, client, proxy, rendering_stats_instrumentation,
+      shared_bitmap_manager, gpu_memory_buffer_manager, compositor_mutator,
+      id));
 }
 
 LayerTreeHostImpl::LayerTreeHostImpl(
@@ -185,6 +183,7 @@ LayerTreeHostImpl::LayerTreeHostImpl(
     RenderingStatsInstrumentation* rendering_stats_instrumentation,
     SharedBitmapManager* shared_bitmap_manager,
     gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager,
+    CompositorMutator* compositor_mutator,
     int id)
     : client_(client),
       proxy_(proxy),
@@ -223,6 +222,7 @@ LayerTreeHostImpl::LayerTreeHostImpl(
       micro_benchmark_controller_(this),
       shared_bitmap_manager_(shared_bitmap_manager),
       gpu_memory_buffer_manager_(gpu_memory_buffer_manager),
+      compositor_mutator_(compositor_mutator),
       id_(id),
       requires_high_res_to_draw_(false),
       is_likely_to_require_a_draw_(false),
@@ -386,6 +386,13 @@ void LayerTreeHostImpl::Animate(base::TimeTicks monotonic_time) {
   AnimateTopControls(monotonic_time);
 }
 
+void LayerTreeHostImpl::ApplyCustomMutations(base::TimeTicks monotonic_time) {
+  // ...
+  compositor_mutator_->ApplyCustomMutations(monotonic_time,
+                                            std::vector<Mutation>());
+  // ...
+}
+
 void LayerTreeHostImpl::PrepareTiles() {
   if (!tile_manager_)
     return;