Add IPC messages to transfer compositor protos

cc/proto/compositor_message.proto

+// 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.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package cc.proto;
+
+// A top level message for serialized state used by the compositor remote
+// channels.
+message CompositorMessage {
+  // One of these is set based on where the message is going to.
+  optional CompositorMessageToMain to_main = 1;
+  optional CompositorMessageToImpl to_impl = 2;
+}
+
+message CompositorMessageToMain {
+  // Placeholder for compositor message data meant for ChannelMain.
+}
+
+message CompositorMessageToImpl {
+  // Placeholder for compositor message data meant for ChannelImpl.
+}