[exp] cc: Introduce cc::CompositorMutator.

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index defe796f66d66461042687c1583d1c89ad80c6d1..7c4651eff36ee0f98832afc8bf93a67eaba5a90d 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -50,6 +50,7 @@ namespace cc {
 
 class CompletionEvent;
 class CompositorFrameMetadata;
+class CompositorMutator;
 class DebugRectHistory;
 class EvictionTilePriorityQueue;
 class FrameRateCounter;
@@ -142,6 +143,7 @@ class CC_EXPORT LayerTreeHostImpl
       RenderingStatsInstrumentation* rendering_stats_instrumentation,
       SharedBitmapManager* shared_bitmap_manager,
       gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager,
+      CompositorMutator* compositor_mutator,
       int id);
   ~LayerTreeHostImpl() override;
 
@@ -213,6 +215,7 @@ class CC_EXPORT LayerTreeHostImpl
   virtual void CommitComplete();
   virtual void Animate(base::TimeTicks monotonic_time);
   virtual void UpdateAnimationState(bool start_ready_animations);
+  void ApplyCustomMutations(base::TimeTicks monotinic_time);
   void ActivateAnimations();
   void MainThreadHasStoppedFlinging();
   void DidAnimateScrollOffset();
@@ -531,9 +534,9 @@ class CC_EXPORT LayerTreeHostImpl
       RenderingStatsInstrumentation* rendering_stats_instrumentation,
       SharedBitmapManager* shared_bitmap_manager,
       gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager,
+      CompositorMutator* compositor_mutator,
       int id);
 
-
   // Virtual for testing.
   virtual void AnimateLayers(base::TimeTicks monotonic_time);
 
@@ -731,6 +734,9 @@ class CC_EXPORT LayerTreeHostImpl
 
   SharedBitmapManager* shared_bitmap_manager_;
   gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager_;
+
+  CompositorMutator* compositor_mutator_;
+
   int id_;
 
   std::set<SwapPromiseMonitor*> swap_promise_monitor_;