Add UNKNOWN values to blimp proto enums

blimp/common/proto/input.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 the input subprotocol.
 //
 // Current definitions are just placeholders and are NOT final.
 // Feel free to modify this interface as necessary during feature work.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 import "common.proto";
 
 package blimp;
 
 message ClickArgs {
   optional CoordinatePair target = 1;
 }
 
 message DragArgs {
   optional CoordinatePair origin = 1;
   optional CoordinatePair destination = 2;
   optional CoordinatePair elastic_overscroll = 3;
 }
 
 message InputMessage {
   enum Type {
+    UNKNOWN = 0;
     CLICK = 1;
     DRAG = 2;
   }
   optional Type type = 1;
 
   optional ClickArgs click = 1000;
   optional DragArgs drag = 1001;
 }