Add basic framework for splitting thread proxy.

cc/trees/channel_main.h

+// Copyright 2015 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 CC_TREES_CHANNEL_MAIN_H_
+#define CC_TREES_CHANNEL_MAIN_H_
+
+#include "cc/base/cc_export.h"
+
+namespace cc {
+
+// ChannelMain and ChannelImpl provide an abstract communication layer for
+// the main and impl side of the compositor.
+//
+// The communication sequence between the 2 sides is:

[enne (OOO), 2015/09/24 22:40:44]

If the second diagram is just the first reversed, you could just make the arrows
point both ways and skip the second one?

[Khushal, 2015/09/24 23:46:50]

That makes more sense. Done.

+//
+//     LayerTreeHost-->ProxyMain-->ChannelMain
+//                                      |
+//                                      |
+//                                 ChannelImpl-->ProxyImpl-->LayerTreeHostImpl
+//
+// LayerTreeHostImpl-->ProxyImpl-->ChannelImpl
+//                                      |
+//                                      |
+//                                 ChannelMain-->ProxyMain-->LayerTreeHost
+
+class CC_EXPORT ChannelMain {
+ public:
+  // Interface for commands sent to the ProxyImpl
+  virtual void SetThrottleFrameProductionOnImpl(bool throttle) = 0;
+  virtual void SetLayerTreeHostClientReadyOnImpl() = 0;

[enne (OOO), 2015/09/24 22:40:44]

I assume these are just a start and you'll add the rest of these in follow-up
patches?

[Khushal, 2015/09/24 23:46:50]

Yes. Just wanted to get set up the framework right now. I'll start adding
methods incrementally.

+
+  virtual ~ChannelMain() {}
+};
+
+}  // namespace cc
+
+#endif  // CC_TREES_CHANNEL_MAIN_H_