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

third_party/WebKit/Source/core/dom/CompositorProxyClientFactory.h

Index: third_party/WebKit/Source/core/dom/CompositorProxyClientFactory.h
diff --git a/third_party/WebKit/Source/core/dom/CompositorProxyClientFactory.h b/third_party/WebKit/Source/core/dom/CompositorProxyClientFactory.h
new file mode 100644
index 0000000000000000000000000000000000000000..de2a759c2dfd022d48577333e8dbce855a6c8f94
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/CompositorProxyClientFactory.h
@@ -0,0 +1,21 @@
+// 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 CompositorProxyClientFactory_h
+#define CompositorProxyClientFactory_h
+
+#include "core/CoreExport.h"
+#include "core/dom/CompositorProxyClient.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class CORE_EXPORT CompositorProxyClientFactory {
+ public:
+  virtual CompositorProxyClient* create(CompositorProxyClient::Type) = 0;
+};
+
+}  // namespace blink
+
+#endif  // CompositorProxyClientFactory_h