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

Source/core/testing/InternalSettings.idl

 /*
  * Copyright (C) 2012 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 35 matching lines @@
     [RaisesException] void setShouldDisplayTrackKind(DOMString kind, boolean enabled);
     [RaisesException] boolean shouldDisplayTrackKind(DOMString trackKind);
     [RaisesException] void setImagesEnabled(boolean enabled);
     [RaisesException] void setDefaultVideoPosterURL(DOMString poster);
 
     // FIXME: cssVariablesEnabled is a real Setting and not a
     // RuntimeEnabledFeature, but it should be changed to be a REF.
     [RaisesException] boolean cssVariablesEnabled();
     [RaisesException] void setCSSVariablesEnabled(boolean enabled);
 
+    // Values must be kept in sync with AcceleratedCompositingForOverflowScrollEnabledMode.
+    const unsigned short AcceleratedCompositingForOverflowScrollDisabled = 0;
+    const unsigned short AcceleratedCompositingForOverflowScrollEnabled = 1;
+    const unsigned short AcceleratedCompositingForOverflowScrollAlwaysOn = 2;
+    [RaisesException] void setAcceleratedCompositingForOverflowScrollEnabledMode(unsigned short enabled);
+
     // FIXME: The following are RuntimeEnabledFeatures and likely
     // cannot be changed after process start.  These setters should
     // be removed or moved onto internals.runtimeFlags:
     void setAuthorShadowDOMForAnyElementEnabled(boolean enabled);
     void setCSSExclusionsEnabled(boolean enabled);
     void setDialogElementEnabled(boolean enabled);
     void setExperimentalShadowDOMEnabled(boolean enabled);
     void setLangAttributeAwareFormControlUIEnabled(boolean enabled);
     void setLazyLayoutEnabled(boolean enabled);
     void setStyleScopedEnabled(boolean enabled);
 };