Clean up the way layout tests force elements to opt in/out of composited scrolling.

Source/core/testing/InternalSettings.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  * Copyright (C) 2013 Apple 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 277 matching lines @@
     InternalSettingsGuardForSettings();
     settings()->setCSSVariablesEnabled(enabled);
 }
 
 bool InternalSettings::cssVariablesEnabled(ExceptionCode& ec)
 {
     InternalSettingsGuardForSettingsReturn(false);
     return settings()->cssVariablesEnabled();
 }
 
+void InternalSettings::setAcceleratedCompositingForOverflowScrollEnabledMode(unsigned enabled, ExceptionCode& ec)
+{
+    InternalSettingsGuardForSettings();
+    settings()->setAcceleratedCompositingForOverflowScrollEnabled(static_cast<AcceleratedCompositingForOverflowScrollEnabledMode>(enabled));
+}
+
 void InternalSettings::setEditingBehavior(const String& editingBehavior, ExceptionCode& ec)
 {
     InternalSettingsGuardForSettings();
     if (equalIgnoringCase(editingBehavior, "win"))
         settings()->setEditingBehaviorType(EditingWindowsBehavior);
     else if (equalIgnoringCase(editingBehavior, "mac"))
         settings()->setEditingBehaviorType(EditingMacBehavior);
     else if (equalIgnoringCase(editingBehavior, "unix"))
         settings()->setEditingBehaviorType(EditingUnixBehavior);
     else if (equalIgnoringCase(editingBehavior, "android"))
@@ skipping 60 matching lines @@
     settings()->setImagesEnabled(enabled);
 }
 
 void InternalSettings::setDefaultVideoPosterURL(const String& url, ExceptionCode& ec)
 {
     InternalSettingsGuardForSettings();
     settings()->setDefaultVideoPosterURL(url);
 }
 
 }