Add 'blink::WebDisplayModeLast' enum element

public/platform/WebDisplayMode.h

 // 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.
 
 #ifndef WebDisplayMode_h
 #define WebDisplayMode_h
 
 namespace blink {
 
 enum WebDisplayMode {
     WebDisplayModeUndefined, // User for override setting (ie. not set).
     WebDisplayModeBrowser,
     WebDisplayModeMinimalUi,
     WebDisplayModeStandalone,
-    WebDisplayModeFullscreen
+    WebDisplayModeFullscreen,
+    WebDisplayModeLast = WebDisplayModeFullscreen
 };
 
 } // namespace blink
 
 #endif // WebDisplayMode_h