Add UNKNOWN values to blimp proto enums

blimp/common/proto/navigation.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 navigation messages.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
@@ skipping 16 matching lines @@
   // If the favicon of the page changed, this should include the serialized
   // bitmap of the favicon.
   optional bytes favicon = 2;
 
   // If the title changed, this will contain the new title string.
   optional string title = 3;
 }
 
 message NavigationMessage {
   enum Type {
+    UNKNOWN = 0;
+
     // Server => Client types.
     NAVIGATION_STATE_CHANGED = 1;
 
     // Client => Server types.
     LOAD_URL = 2;
     GO_BACK = 3;
     GO_FORWARD = 4;
     RELOAD = 5;
   }
 
   optional Type type = 1;
 
   optional NavigationStateChangeMessage navigation_state_change = 1000;
   optional LoadUrlMessage load_url = 1001;
 }