Add a setting to disable threaded scrolling

public/web/WebSettings.h

 /*
  * Copyright (C) 2009 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 188 matching lines @@
     // event handlers specified. User can also trigger click handlers for such
     // elements using SPACE or ENTER keys.
     virtual void setSpatialNavigationEnabled(bool) = 0;
     virtual void setStandardFontFamily(const WebString&, UScriptCode = USCRIPT_COMMON) = 0;
     virtual void setSupportDeprecatedTargetDensityDPI(bool) = 0;
     virtual void setSupportsMultipleWindows(bool) = 0;
     virtual void setSyncXHRInDocumentsEnabled(bool) = 0;
     virtual void setTextAreasAreResizable(bool) = 0;
     virtual void setTextAutosizingEnabled(bool) = 0;
     virtual void setAccessibilityFontScaleFactor(float) = 0;
+    virtual void setThreadedScrollingEnabled(bool) = 0;
     virtual void setTouchDragDropEnabled(bool) = 0;
     virtual void setTouchEditingEnabled(bool) = 0;
     virtual void setUnifiedTextCheckerEnabled(bool) = 0;
     virtual void setUnsafePluginPastingEnabled(bool) = 0;
     virtual void setUseLegacyBackgroundSizeShorthandBehavior(bool) = 0;
     virtual void setUseSolidColorScrollbars(bool) = 0;
     virtual void setUseWideViewport(bool) = 0;
     virtual void setUsesEncodingDetector(bool) = 0;
     virtual void setV8CacheOptions(V8CacheOptions) = 0;
     virtual void setV8ScriptStreamingEnabled(bool) = 0;
@@ skipping 10 matching lines @@
     virtual void setWideViewportQuirkEnabled(bool) = 0;
     virtual void setXSSAuditorEnabled(bool) = 0;
 
 protected:
     ~WebSettings() { }
 };
 
 } // namespace blink
 
 #endif