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

Source/core/testing/InternalSettings.h

 /*
  * 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 92 matching lines @@
     void setTouchEventEmulationEnabled(bool, ExceptionCode&);
     void setUsesOverlayScrollbars(bool, ExceptionCode&);
 
     void setShouldDisplayTrackKind(const String& kind, bool, ExceptionCode&);
     bool shouldDisplayTrackKind(const String& kind, ExceptionCode&);
 
     // cssVariablesEnabled is not backed by RuntimeEnabledFeatures, but should be.
     bool cssVariablesEnabled(ExceptionCode&);
     void setCSSVariablesEnabled(bool, ExceptionCode&);
 
+    // This must be kept in sync with AcceleratedCompositingForOverflowScrollEnabledMode.
+    enum {
+        AcceleratedCompositingForOverflowScrollDisabled = 0,
+        AcceleratedCompositingForOverflowScrollEnabled = 1,
+        AcceleratedCompositingForOverflowScrollAlwaysOn = 2
+    };
+    void setAcceleratedCompositingForOverflowScrollEnabledMode(unsigned enabled, ExceptionCode&);
+
     // 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(bool);
     void setCSSExclusionsEnabled(bool);
     void setDialogElementEnabled(bool);
     void setExperimentalShadowDOMEnabled(bool);
     void setLangAttributeAwareFormControlUIEnabled(bool);
     void setLazyLayoutEnabled(bool);
     void setStyleScopedEnabled(bool);
 
 private:
     explicit InternalSettings(Page*);
 
     Settings* settings() const;
     Page* page() const { return m_page; }
     static const char* supplementName();
 
     Page* m_page;
     Backup m_backup;
 };
 
 } // namespace WebCore
 
 #endif