[exp] cc: Introduce cc::CompositorMutator.

cc/scheduler/scheduler_unittest.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/scheduler/scheduler.h"
 
 #include <string>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 94 matching lines @@
   // SchedulerClient implementation.
   void WillBeginImplFrame(const BeginFrameArgs& args) override {
     PushAction("WillBeginImplFrame");
   }
   void ScheduledActionSendBeginMainFrame() override {
     PushAction("ScheduledActionSendBeginMainFrame");
   }
   void ScheduledActionAnimate() override {
     PushAction("ScheduledActionAnimate");
   }
+  void ScheduledActionCustomMutate() override {}
   DrawResult ScheduledActionDrawAndSwapIfPossible() override {
     PushAction("ScheduledActionDrawAndSwapIfPossible");
     num_draws_++;
     DrawResult result =
         draw_will_happen_ ? DRAW_SUCCESS : DRAW_ABORTED_CHECKERBOARD_ANIMATIONS;
     bool swap_will_happen =
         draw_will_happen_ && swap_will_happen_if_draw_happens_;
     if (swap_will_happen) {
       scheduler_->DidSwapBuffers();
 
@@ skipping 2104 matching lines @@
   client_->Reset();
 
   task_runner().RunPendingTasks();  // Run posted deadline.
   EXPECT_ACTION("SetNeedsBeginFrames(false)", client_, 0, 2);
   EXPECT_ACTION("SendBeginMainFrameNotExpectedSoon", client_, 1, 2);
   client_->Reset();
 }
 
 }  // namespace
 }  // namespace cc