Introduce AnimationWorkletProxyClient and necessary plumbing to get it in worklet messaging proxy.

third_party/WebKit/Source/platform/graphics/CompositorMutator.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 CompositorMutator_h
 #define CompositorMutator_h
 
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class CompositorMutableStateProvider;
 
 class CompositorMutator : public GarbageCollectedFinalized<CompositorMutator> {
  public:
   virtual ~CompositorMutator() {}
 
   DEFINE_INLINE_VIRTUAL_TRACE() {}
 
   // Called from compositor thread to run the animation frame callbacks from all
-  // connected CompositorWorkers.
+  // connected CompositorWorkers and AnimationWorklets.
   // Returns true if any animation callbacks requested an animation frame
   // (i.e. should be reinvoked next frame).
   virtual bool mutate(double monotonicTimeNow,
                       CompositorMutableStateProvider*) = 0;
 };
 
 }  // namespace blink
 
 #endif  // CompositorMutator_h