compositor-worker: Initialize CW machinery plumbing to compositor and fire CW rAF callbacks.

cc/animation/layer_tree_mutator.h

 // Copyright 2016 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.
 
 #ifndef CC_ANIMATION_LAYER_TREE_MUTATOR_H_
 #define CC_ANIMATION_LAYER_TREE_MUTATOR_H_
 
 #include "base/callback_forward.h"
+#include "base/time/time.h"
 #include "cc/base/cc_export.h"
 
 namespace cc {
 
+class LayerTreeImpl;
+
+class LayerTreeMutatorClient {
+ public:
+  // This is necessary because it forces an impl frame. We couldn't, for
+  // example, just assume that we will "always mutate" and early-out in the
+  // mutator if there was nothing to do because we won't always be producing
+  // impl frames.
+  virtual void SetNeedsMutate() = 0;
+};
+
 class CC_EXPORT LayerTreeMutator {
  public:
   virtual ~LayerTreeMutator() {}
 
+  // Returns true if the mutator should be rescheduled.
+  virtual bool Mutate(base::TimeTicks now) = 0;
+  virtual void SetClient(LayerTreeMutatorClient* client) = 0;
+
   // Returns a callback which is responsible for applying layer tree mutations
   // to DOM elements.
   virtual base::Closure TakeMutations() = 0;
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_LAYER_TREE_MUTATOR_H_