Add glue between the client and engine for Blimp

blimp/common/proto/render_widget.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.
 //
-// Message definitions for browser control messages.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
-message ControlMessage {
+package blimp;
+
+message RenderWidgetMessage {
   enum Type {
-    UNKNOWN = 0;
+    // TODO(dtrainor): Leaving this a bit open in case we need to send other
+    // types of RenderWidget messages (like shutdown).
 
-    // Client <=> Server types.
-    CREATE_TAB = 1;
-    CLOSE_TAB = 2;
+    // Engine => Client types.
+    INITIALIZE = 1;
   }
+
   optional Type type = 1;
-}
+  optional uint32 render_widget_id = 2;
+}