Add download button to MediaDocument on Android.

third_party/WebKit/public/web/WebRuntimeFeatures.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 70 matching lines @@
     BLINK_EXPORT static void enableFastMobileScrolling(bool);
 
     BLINK_EXPORT static void enableFileSystem(bool);
 
     BLINK_EXPORT static void enableImageColorProfiles(bool);
 
     BLINK_EXPORT static void enableSubpixelFontScaling(bool);
 
     BLINK_EXPORT static void enableMediaCapture(bool);
 
+    BLINK_EXPORT static void enableMediaDocumentDownloadButton(bool);
+
     BLINK_EXPORT static void enableMediaRecorder(bool);
 
     BLINK_EXPORT static void enableMediaSource(bool);
 
     BLINK_EXPORT static void enableNotificationActionIcons(bool);
 
     BLINK_EXPORT static void enableNotificationConstructor(bool);
 
     BLINK_EXPORT static void enableNotifications(bool);
 
@@ skipping 65 matching lines @@
     // (crbug.com/543198).
     BLINK_EXPORT static bool isServiceWorkerExtendableMessageEventEnabled();
 
 private:
     WebRuntimeFeatures();
 };
 
 } // namespace blink
 
 #endif