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.
+//
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+import "compositor.proto";
+import "input.proto";
+
+package blimp;
+
+message RenderWidgetMessage {
+  enum Type {
+    // TODO(dtrainor): Leaving this a bit open in case we need to send other
+    // types of RenderWidget messages (like shutdown).
+
+    // Engine => Client types.
+    INITIALIZE = 1;
+  }
+
+  optional Type type = 1;
+  optional uint32 render_widget_id = 2;
+}