Add a setting to disable threaded scrolling

Source/core/frame/Settings.in

 # Defines properties which are available on the Settings object.
 #
 # Please think carefully before adding a new Setting.  Some questions to
 # consider are:
 # - Should this be a RuntimeEnabledFeature instead? Settings are for things
 #   which we support either values of at runtime.  Features are set at renderer
 #   process startup and are never changed. Features also tend to be set to a
 #   value based on the platform or the stability of the code in question, where
 #   as settings both codepaths need to be stable.
 # - How will you ensure test coverage of all relevant values of your setting?
@@ skipping 100 matching lines @@
 allowDisplayOfInsecureContent initial=true
 allowRunningOfInsecureContent initial=true
 # FIXME: Remove this temporary flag. See crbug.com/366483 for the target
 # milestone.
 allowConnectingInsecureWebSocket initial=false
 mediaControlsOverlayPlayButtonEnabled initial=false
 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
 
 shouldRespectImageOrientation initial=false
 
 # Limited use by features which behave differently depending on the input
 # devices available.  For example, the pointer and hover media queries.
 # Note that we need to be careful when basing behavior or UI on this -
 # just because a device is present doesn't mean the user cares about it
@@ skipping 159 matching lines @@
 # has pointerType coarse *and* fine).
 availablePointerTypes type=int, initial=PointerTypeNone, invalidate=MediaQuery
 availableHoverTypes type=int, initial=HoverTypeNone, invalidate=MediaQuery
 
 # These values specify properties of the user's primary pointing device only.
 primaryPointerType type=PointerType, initial=PointerTypeNone, invalidate=MediaQuery
 primaryHoverType type=HoverType, initial=HoverTypeNone, invalidate=MediaQuery
 
 accessibilityEnabled initial=false
 inlineTextBoxAccessibilityEnabled initial=false