Enable new multicol for testing and experimental web platform features.

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?
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 shouldClearDocumentBackground initial=true
 
 textAreasAreResizable initial=false, invalidate=Style
 acceleratedCompositingEnabled initial=true, invalidate=AcceleratedCompositing
 
 shrinksStandaloneImagesToFit initial=true
 
 offlineWebApplicationCacheEnabled initial=true
 usesEncodingDetector initial=false
 allowScriptsToCloseWindows initial=false
-regionBasedColumnsEnabled initial=false
 
 # FIXME: This should really be disabled by default as it makes platforms that
 # don't support the feature download files they can't use by.
 # Leaving enabled for now to not change existing behavior.
 downloadableBinaryFontsEnabled initial=true
 
 xssAuditorEnabled initial=false
 unsafePluginPastingEnabled initial=false
 treatIPAddressAsDomain initial=false
 
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 useMobileViewportStyle initial=false, invalidate=ViewportRule
 
 # User style overrides for captions and subtitles
 textTrackBackgroundColor type=String
 textTrackFontFamily type=String
 textTrackFontStyle type=String
 textTrackFontVariant type=String
 textTrackTextColor type=String
 textTrackTextShadow type=String
 textTrackTextSize type=String