[exp] cc: Introduce cc::CompositorMutator.

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 6e1a0d9a9d6c15afe87bcff657bd44a397fcc561..54a0c83d66de96241cf23eb3c6a098888c9462ff 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -952,9 +952,9 @@ void ThreadProxy::ScheduledActionAnimate() {
     return;
   }
 
-  impl().animation_time =
+  base::TimeTicks animation_time =
       impl().layer_tree_host_impl->CurrentBeginFrameArgs().frame_time;
-  impl().layer_tree_host_impl->Animate(impl().animation_time);
+  impl().layer_tree_host_impl->Animate(animation_time);
 
   // If animations are not visible, update the state now as
   // ScheduledActionDrawAndSwapIfPossible will never be called.
@@ -963,6 +963,14 @@ void ThreadProxy::ScheduledActionAnimate() {
   }
 }
 
+void ThreadProxy::ScheduledActionCustomMutate() {
+  // ...
+  base::TimeTicks frame_time =
+      impl().layer_tree_host_impl->CurrentBeginFrameArgs().frame_time;
+  impl().layer_tree_host_impl->ApplyCustomMutations(frame_time);
+  // ...
+}
+
 void ThreadProxy::ScheduledActionCommit() {
   TRACE_EVENT0("cc", "ThreadProxy::ScheduledActionCommit");
   DCHECK(IsImplThread());