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

third_party/WebKit/Source/web/CompositorAnimator.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 CompositorAnimator_h
+#define CompositorAnimator_h
+
+#include "platform/graphics/CompositorMutableStateProvider.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class CompositorAnimator : public GarbageCollectedMixin {
+ public:
+  // Runs the animation frame callback for the frame starting at the given time.
+  // Returns true if another animation frame was requested (i.e. should be
+  // reinvoked next frame).
+  virtual bool mutate(double monotonicTimeNow,
+                      CompositorMutableStateProvider*) = 0;
+  DEFINE_INLINE_VIRTUAL_TRACE() {}
+};
+
+}  // namespace blink
+
+#endif  // CompositorAnimator_h