Presentation API: add WebRuntimeFeatures to enable/disable the API.

Source/web/WebRuntimeFeatures.cpp

 /*
  * 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 316 matching lines @@
 void WebRuntimeFeatures::enableWebVR(bool enable)
 {
     RuntimeEnabledFeatures::setWebVREnabled(enable);
 }
 
 void WebRuntimeFeatures::enableNewMediaPlaybackUi(bool enable)
 {
     RuntimeEnabledFeatures::setNewMediaPlaybackUiEnabled(enable);
 }
 
+void WebRuntimeFeatures::enablePresentationAPI(bool enable)
+{
+    RuntimeEnabledFeatures::setPresentationEnabled(enable);
+}
+
 } // namespace blink