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..12ed3bdf73414e4868ef48c889e8fcdf28e35c16 100644
--- a/cc/proto/compositor_message.proto
+++ b/cc/proto/compositor_message.proto
@@ -21,6 +21,11 @@ message CompositorMessage {
   // Frame Update.
   optional LayerTreeHost layer_tree_host = 3;
 
+  // Client -> Engine
+  // Sent from the client to the engine when a frame update was processed
+  // successfully on the client.
+  optional bool frame_ack = 4;
+
   // One of these is set based on where the message is going to.
   optional CompositorMessageToMain to_main = 1;
   optional CompositorMessageToImpl to_impl = 2;