Remove ScrollableArea and Scrollbar

sky/engine/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?
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 selectionIncludesAltImageText initial=false
 
 # Used by the android_webview to support a horizontal height auto-sizing
 # mode.
 forceZeroLayoutHeight initial=false
 
 # Touch based text selection and editing on desktop.
 # crbug.com/304873 tracks removal once it's been enabled on all platforms.
 touchEditingEnabled initial=false
 
-useSolidColorScrollbars initial=true
-
 mainFrameClipsContent initial=true
 
 # Presumably used by tests?  Unclear.
 useWideViewport initial=true
 loadWithOverviewMode initial=true
 
 # Only set by Layout Tests.
 mediaTypeOverride type=String, initial="screen", invalidate=MediaType
 
 # loadsImagesAutomatically only suppresses the network load of
 # the image URL. A cached image will still be rendered if requested.
 loadsImagesAutomatically initial=false, invalidate=ImageLoading
 imagesEnabled initial=true, invalidate=ImageLoading
 
 # V8 supports different types of caching. Used by V8 bindings.
 v8CacheOptions type=V8CacheOptions, initial=V8CacheOptionsOff
 
 # These values are bit fields for the properties of available pointing devices
 # and may take on multiple values (e.g. laptop with touchpad and touchscreen
 # 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