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

Source/core/page/Settings.h

Index: Source/core/page/Settings.h
diff --git a/Source/core/page/Settings.h b/Source/core/page/Settings.h
index 7ef12c80fd873454a40ddfbf68988d005cf6485d..54fecd9cbc42090647f0c35b840e66dbde0bc75c 100644
--- a/Source/core/page/Settings.h
+++ b/Source/core/page/Settings.h
@@ -56,6 +56,12 @@ enum TextDirectionSubmenuInclusionBehavior {
     TextDirectionSubmenuAlwaysIncluded
 };
 
+enum AcceleratedCompositingForOverflowScrollEnabledMode {
+    AcceleratedCompositingForOverflowScrollDisabled = 0,
+    AcceleratedCompositingForOverflowScrollEnabled = 1,
+    AcceleratedCompositingForOverflowScrollAlwaysOn = 2
+};
+
 // UScriptCode uses -1 and 0 for UScriptInvalidCode and UScriptCommon.
 // We need to use -2 and -3 for empty value and deleted value.
 struct UScriptCodeHashTraits : WTF::GenericHashTraits<int> {