content/blimp: Set up hooks for enabling LTHRemote in the renderer.

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 0697a18611d88352997ad786d28c1445081bd202..41f3f51cb9183530c64fefa0cbfbf129e9b5dde6 100644
--- a/cc/proto/compositor_message.proto
+++ b/cc/proto/compositor_message.proto
@@ -21,6 +21,10 @@ message CompositorMessage {
   // Frame Update.
   optional LayerTreeHost layer_tree_host = 3;
 
+  // Client -> Engine
+  // Frame Ack.
+  optional bool frame_ack = 4;

[Wez, 2016/10/13 16:53:52]

Can you clarify what values this holds?  What does a true vs false ACK mean?

[Khushal, 2016/10/13 18:47:28]

The value can never be false. I suppose it should be an enum for the message
type.

But I'm going to kill these protos in cc as soon as we remove the
RemoteProtoChannel, so its going to be much clearer and well-structured when
these move into blimp.

+
   // One of these is set based on where the message is going to.
   optional CompositorMessageToMain to_main = 1;
   optional CompositorMessageToImpl to_impl = 2;