cc:: Add remote mode to the compositor

cc/proto/compositor_message.proto

Index: cc/proto/compositor_message.proto
diff --git a/cc/proto/compositor_message.proto b/cc/proto/compositor_message.proto
index 98c781c703f29bda1298333a80b33d0220e52ef9..3d67ba3bf93f2bd9a1dc7101b086179ddc3674cd 100644
--- a/cc/proto/compositor_message.proto
+++ b/cc/proto/compositor_message.proto
@@ -6,6 +6,9 @@ syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
+import "compositor_message_to_impl.proto";
+import "compositor_message_to_main.proto";
+
 package cc.proto;
 
 // A top level message for serialized state used by the compositor remote
@@ -15,11 +18,3 @@ message CompositorMessage {
   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.
-}