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

third_party/WebKit/Source/web/CompositorProxyClientFactoryImpl.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 CompositorProxyClientFactoryImpl_h
+#define CompositorProxyClientFactoryImpl_h
+
+#include "core/dom/CompositorProxyClientFactory.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class CompositorMutatorImpl;
+
+class CompositorProxyClientFactoryImpl final
+    : public GarbageCollectedFinalized<CompositorProxyClientFactoryImpl>,
+      public CompositorProxyClientFactory {
+ public:
+  explicit CompositorProxyClientFactoryImpl(CompositorMutatorImpl*);
+  CompositorProxyClient* create(CompositorProxyClient::Type) override;

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

Nit: It's nice to annotate this with something like:

// CompositorProxyClientFactory overrides:

Since many classes also have a factory create() method, so it's a bit confusing
here.

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

I ended up removing the factory. So this is no longer an issue.

+
+  DEFINE_INLINE_TRACE() {}
+
+ private:
+  CrossThreadPersistent<CompositorMutatorImpl> m_mutator;

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

This code has a lot of CrossThreadPersistents...

Is there any documentation that describes how all these different pieces work
together?

[haraken, 2016/11/29 02:35:36]

Yeah, I agree that this needs to be CrossThreadPersistent but am wondering why
we need to have m_mutator in so many places.

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

OK. I realized the factory is not really doing much and may not be worth
introducing yet another CrossThreadPersistent for it. so I simply went with
existing design (a method on WebViewImp) but only added a new param to it.

I just created a doc for the animation worklet design. It is very basebone at
this point but I added a diagram that captures changes related to this patch.

https://docs.google.com/a/chromium.org/document/d/1MdpvGtnK_A2kTzLeTd07NUevMO...

+};
+
+}  // namespace blink
+
+#endif  // CompositorProxyClientFactoryImpl_h