[exp] cc: Introduce cc::CompositorMutator.

cc/scheduler/scheduler_state_machine.h

Index: cc/scheduler/scheduler_state_machine.h
diff --git a/cc/scheduler/scheduler_state_machine.h b/cc/scheduler/scheduler_state_machine.h
index a411022c003c1d8932554558b91f4759056bca9c..76ec9417eece1fdd24bb20394de7dc6341840cd4 100644
--- a/cc/scheduler/scheduler_state_machine.h
+++ b/cc/scheduler/scheduler_state_machine.h
@@ -103,6 +103,7 @@ class CC_EXPORT SchedulerStateMachine {
   enum Action {
     ACTION_NONE,
     ACTION_ANIMATE,
+    ACTION_CUSTOM_MUTATE,
     ACTION_SEND_BEGIN_MAIN_FRAME,
     ACTION_COMMIT,
     ACTION_ACTIVATE_SYNC_TREE,
@@ -172,6 +173,9 @@ class CC_EXPORT SchedulerStateMachine {
   void SetNeedsAnimate();
   bool needs_animate() const { return needs_animate_; }
 
+  void SetNeedsCustomMutate();
+  bool needs_custom_mutate() const { return needs_custom_mutate_; }
+
   // Indicates that prepare-tiles is required. This guarantees another
   // PrepareTiles will occur shortly (even if no redraw is required).
   void SetNeedsPrepareTiles();
@@ -276,6 +280,7 @@ class CC_EXPORT SchedulerStateMachine {
   bool PendingActivationsShouldBeForced() const;
 
   bool ShouldAnimate() const;
+  bool ShouldCustomMutate() const;
   bool ShouldBeginOutputSurfaceCreation() const;
   bool ShouldDraw() const;
   bool ShouldActivatePendingTree() const;
@@ -285,6 +290,7 @@ class CC_EXPORT SchedulerStateMachine {
 
   void AdvanceCurrentFrameNumber();
   bool HasAnimatedThisFrame() const;
+  bool HasCustomMutatedThisFrame() const;
   bool HasSentBeginMainFrameThisFrame() const;
   bool HasRequestedSwapThisFrame() const;
   bool HasSwappedThisFrame() const;
@@ -306,6 +312,7 @@ class CC_EXPORT SchedulerStateMachine {
   int commit_count_;
   int current_frame_number_;
   int last_frame_number_animate_performed_;
+  int last_frame_number_custom_mutate_performed_;
   int last_frame_number_swap_performed_;
   int last_frame_number_swap_requested_;
   int last_frame_number_begin_main_frame_sent_;
@@ -320,6 +327,7 @@ class CC_EXPORT SchedulerStateMachine {
   int pending_swaps_;
   bool needs_redraw_;
   bool needs_animate_;
+  bool needs_custom_mutate_;
   bool needs_prepare_tiles_;
   bool needs_commit_;
   bool inside_poll_for_anticipated_draw_triggers_;