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

Source/core/testing/InternalSettings.cpp

Index: Source/core/testing/InternalSettings.cpp
diff --git a/Source/core/testing/InternalSettings.cpp b/Source/core/testing/InternalSettings.cpp
index e0183786ffece6ea0787beb2bbb17e107fa017eb..c0cff264abeb067ba5a52dc40a8bd89a2a432468 100644
--- a/Source/core/testing/InternalSettings.cpp
+++ b/Source/core/testing/InternalSettings.cpp
@@ -295,6 +295,12 @@ bool InternalSettings::cssVariablesEnabled(ExceptionCode& ec)
     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();