Allow enabling "CSSViewport" runtime feature outside Blink

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 121 matching lines @@
     BLINK_EXPORT static void enableLayerSquashing(bool);
 
     BLINK_EXPORT static void enableShowModalDialog(bool);
 
     BLINK_EXPORT static void enableLaxMixedContentChecking(bool);
 
     BLINK_EXPORT static void enableCredentialManagerAPI(bool);
 
     BLINK_EXPORT static void enableTextBlobs(bool);
 
+    BLINK_EXPORT static void setCSSViewportEnabled(bool);
+
 private:
     WebRuntimeFeatures();
 };
 
 } // namespace blink
 
 #endif