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

third_party/WebKit/Source/web/AnimationWorkletProxyClientImpl.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 AnimationWorkletProxyClientImpl_h
+#define AnimationWorkletProxyClientImpl_h
+
+#include "core/dom/AnimationWorkletProxyClient.h"
+#include "platform/graphics/CompositorAnimator.h"
+#include "platform/heap/Handle.h"
+#include "wtf/Noncopyable.h"
+
+namespace blink {
+
+class CompositorMutatorImpl;
+
+// Mediates between one Animator and the associated CompositorMutatorImpl. There
+// is one AnimationWorkletProxyClientImpl per Animator but there may be multiple
+// for a given mutator and animatorWorklet.
+//
+// Should be accessed only on the compositor thread.

[dcheng, 2016/11/28 21:17:31]

This comment seems to be directly in opposition to the DCHECK() in the ctor that
it's executing on the main thread...

[majidvp, 2017/01/20 15:48:57]

I clarified the comment. This is similar to a worker client as it is 
constructed on main thread but its methods are used in the worklet
backing thread which in this case is not the main thread.

Added DCHECK per haraken@ suggestions that asserts the above statement.

+class AnimationWorkletProxyClientImpl final
+    : public GarbageCollectedFinalized<AnimationWorkletProxyClientImpl>,
+      public AnimationWorkletProxyClient,
+      public CompositorAnimator {
+  WTF_MAKE_NONCOPYABLE(AnimationWorkletProxyClientImpl);
+  USING_GARBAGE_COLLECTED_MIXIN(AnimationWorkletProxyClientImpl);
+
+ public:
+  AnimationWorkletProxyClientImpl(CompositorMutatorImpl*);

[dcheng, 2016/11/28 21:17:32]

Nit: explicit

Or even better, add |CompositorMutatorImpl*| in the future CL where it will be
used (since it's not obvious why this needs to be a cross-thread persistent)

+  DECLARE_VIRTUAL_TRACE();
+
+  // CompositorAnimator:
+  bool mutate(double monotonicTimeNow,
+              CompositorMutableStateProvider*) override;
+
+  // CompositorProxyClient:
+  void registerCompositorProxy(CompositorProxy*) override;
+  void unregisterCompositorProxy(CompositorProxy*) override;
+
+ private:
+  CrossThreadPersistent<CompositorMutatorImpl> m_mutator;
+  HeapHashSet<WeakMember<CompositorProxy>> m_proxies;
+};
+
+}  // namespace blink
+
+#endif  // AnimationWorkletProxyClientImpl_h