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

Source/core/testing/InternalSettings.h

Index: Source/core/testing/InternalSettings.h
diff --git a/Source/core/testing/InternalSettings.h b/Source/core/testing/InternalSettings.h
index 5bb84ab469f86c1478c930018e30a79754119b98..4f9e0d53bd4e40cb0c13c4bb2f1c86dec4d658df 100644
--- a/Source/core/testing/InternalSettings.h
+++ b/Source/core/testing/InternalSettings.h
@@ -110,6 +110,14 @@ public:
     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: