cc/blimp: Set up the framework for state serialization.

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 3d67ba3bf93f2bd9a1dc7101b086179ddc3674cd..0697a18611d88352997ad786d28c1445081bd202 100644
--- a/cc/proto/compositor_message.proto
+++ b/cc/proto/compositor_message.proto
@@ -8,12 +8,19 @@ option optimize_for = LITE_RUNTIME;
 
 import "compositor_message_to_impl.proto";
 import "compositor_message_to_main.proto";
+import "layer_tree_host.proto";
 
 package cc.proto;
 
 // A top level message for serialized state used by the compositor remote
 // channels.
+// TODO(khushalsagar): Remove CompositorMessage and stop using
+// RemoteProtoChannel to plumb these to/from the browser. See crbug.com/648442
 message CompositorMessage {
+  // Engine -> Client
+  // Frame Update.
+  optional LayerTreeHost layer_tree_host = 3;
+
   // One of these is set based on where the message is going to.
   optional CompositorMessageToMain to_main = 1;
   optional CompositorMessageToImpl to_impl = 2;