Add a setting to disable threaded scrolling

Source/core/frame/Settings.in

Index: Source/core/frame/Settings.in
diff --git a/Source/core/frame/Settings.in b/Source/core/frame/Settings.in
index 8a220762727044e20ff0143ec644e59f8dd31402..be2c99d965b0e2f65bc753560a7a3ef3ace30302 100644
--- a/Source/core/frame/Settings.in
+++ b/Source/core/frame/Settings.in
@@ -118,6 +118,11 @@ mediaPlaybackRequiresUserGesture initial=false
 
 scrollAnimatorEnabled initial=true
 
+# Used to disable threaded, compositor scrolling for testing purposes.
+# crbug.com/410974 tracks removal once alternative solutions for selective
+# main thread scrolling are supported.
+threadedScrollingEnabled initial=true, invalidate=Style
+
 # Used in layout tests for gesture tap highlights. Makes the highlights square
 # (rather than rounded) to make it possible to reftest the results.
 mockGestureTapHighlightsEnabled initial=false