Add UNKNOWN values to blimp proto enums

blimp/common/proto/control.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 {
   enum Type {
+    UNKNOWN = 0;
+
     // Client <=> Server types.
     CREATE_TAB = 1;
     CLOSE_TAB = 2;
   }
   optional Type type = 1;
 }