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

Source/core/testing/InternalSettings.idl

Index: Source/core/testing/InternalSettings.idl
diff --git a/Source/core/testing/InternalSettings.idl b/Source/core/testing/InternalSettings.idl
index 30c0d911e5d9bbf41d0b038abf88963a65c23adb..9caa54df5c85b29e628531f51f381533bdc28bf0 100644
--- a/Source/core/testing/InternalSettings.idl
+++ b/Source/core/testing/InternalSettings.idl
@@ -53,6 +53,12 @@
     [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: