[exp] cc: Introduce cc::CompositorMutator.

cc/trees/thread_proxy.h

Index: cc/trees/thread_proxy.h
diff --git a/cc/trees/thread_proxy.h b/cc/trees/thread_proxy.h
index 51cc18cd95fc6c89d86f6b48c38cb818ea64a32b..bb9a86e9b56dbf99772ad2f2896915e1e7cddd00 100644
--- a/cc/trees/thread_proxy.h
+++ b/cc/trees/thread_proxy.h
@@ -131,8 +131,6 @@ class CC_EXPORT ThreadProxy : public Proxy,
 
     bool input_throttled_until_commit;
 
-    base::TimeTicks animation_time;
-
     // Whether a commit has been completed since the last time animations were
     // ticked. If this happens, we need to animate again.
     bool did_commit_after_animating;
@@ -217,6 +215,7 @@ class CC_EXPORT ThreadProxy : public Proxy,
   DrawResult ScheduledActionDrawAndSwapIfPossible() override;
   DrawResult ScheduledActionDrawAndSwapForced() override;
   void ScheduledActionAnimate() override;
+  void ScheduledActionCustomMutate() override;
   void ScheduledActionCommit() override;
   void ScheduledActionActivateSyncTree() override;
   void ScheduledActionBeginOutputSurfaceCreation() override;