Convert scroll anchoring flag to runtime-enabled feature.

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 143 matching lines @@
 
     BLINK_EXPORT static void enableUnsafeES3APIs(bool);
 
     BLINK_EXPORT static void enableWebVR(bool);
 
     BLINK_EXPORT static void enableNewMediaPlaybackUi(bool);
 
     BLINK_EXPORT static void enablePresentationAPI(bool);
 
     BLINK_EXPORT static void enableWebFontsIntervention(bool);
+
+    BLINK_EXPORT static void enableScrollAnchoring(bool);
 private:
     WebRuntimeFeatures();
 };
 
 } // namespace blink
 
 #endif