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/heap/Handle.h"
+#include "web/CompositorAnimator.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.
+//
+// This is constructed on the main thread but it is used in the worklet backing
+// thread i.e., compositor thread.
+class AnimationWorkletProxyClientImpl final
+    : public AnimationWorkletProxyClient,
+      public CompositorAnimator {
+  WTF_MAKE_NONCOPYABLE(AnimationWorkletProxyClientImpl);
+  USING_GARBAGE_COLLECTED_MIXIN(AnimationWorkletProxyClientImpl);
+
+ public:
+  explicit AnimationWorkletProxyClientImpl(CompositorMutatorImpl*);
+  DECLARE_VIRTUAL_TRACE();
+
+  // CompositorAnimator:
+  // This method is invoked in compositor thread
+  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