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

third_party/WebKit/Source/core/dom/AnimationWorkletProxyClient.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 AnimationWorkletProxyClient_h
+#define AnimationWorkletProxyClient_h
+
+#include "core/CoreExport.h"
+#include "core/dom/CompositorProxyClient.h"
+#include "wtf/Noncopyable.h"
+
+namespace blink {
+
+class CORE_EXPORT AnimationWorkletProxyClient : public CompositorProxyClient {
+  WTF_MAKE_NONCOPYABLE(AnimationWorkletProxyClient);
+
+ public:
+  AnimationWorkletProxyClient() {}
+  DEFINE_INLINE_VIRTUAL_TRACE() { CompositorProxyClient::trace(visitor); }
+};
+
+}  // namespace blink
+
+#endif  // AnimationWorkletProxyClient_h