Add UNKNOWN values to blimp proto enums

blimp/common/proto/blimp_message.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.
 //
 // Contains the BlimpMessage proto which frames all messages sent over Blimp
 // subchannels. BlimpMessage protos are serialized and transmitted over the
 // wire to the Blimplet server.
 //
 // Each BlimpMessage has a few identifying fields which provide the browser
 // session and tab ID as context. The message details are stored in a
@@ skipping 15 matching lines @@
 
 import "control.proto";
 import "compositor.proto";
 import "input.proto";
 import "navigation.proto";
 
 package blimp;
 
 message BlimpMessage {
   enum Type {
-    COMPOSITOR = 0;
-    INPUT = 1;
-    CONTROL = 2;
-    NAVIGATION = 3;
+    UNKNOWN = 0;
+    COMPOSITOR = 1;
+    INPUT = 2;
+    CONTROL = 3;
+    NAVIGATION = 4;
   }
   // Identifies the feature type of this message.
   // The feature-specific contents are contained in optional fields of the same
   // name (example: use |compositor| field for type=COMPOSITOR.)
   optional Type type = 1;
 
   // Uniquely identifies the Blimp session that originated this message.
   // Session IDs are invalidated whenever new sessions are created.
   // If a message's |session_id| does not match the client's session ID,
   // then the message may have originated from a discarded session and can be
   // safely ignored.
   optional int32 session_id = 2;
 
   // ID of the tab that is referenced by this message.
   // Messages that are tab-agnostic may leave this field unset.
   optional int32 target_tab_id = 3;
 
   // Feature-specific messages follow.
   // Only one of these fields may be set per BlimpMessage.
   // TODO(kmarshall): use a 'oneof' union when it's supported in Chromium.
   optional CompositorMessage compositor = 1000;
   optional InputMessage input = 1001;
   optional ControlMessage control = 1002;
   optional NavigationMessage navigation = 1003;
 }