Read the default min-width value from the UA style sheets

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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 # Number of pixels below which 2D canvas is rendered in software
 # even if hardware acceleration is enabled.
 # Hardware acceleration is useful for large canvases where it can avoid the
 # pixel bandwidth between the CPU and GPU. But GPU acceleration comes at
 # a price - extra back-buffer and texture copy. Small canvases are also
 # widely used for stylized fonts. Anti-aliasing text in hardware at that
 # scale is generally slower. So below a certain size it is better to
 # draw canvas in software.
 minimumAccelerated2dCanvasSize type=int, initial=257*256
 
-# The layout width used with --enable-viewport when no viewport meta tag was defined.
-# FIXME: This should get the value from the @viewport UA sheet instead.
-layoutFallbackWidth type=int, initial=980
-
 minimumFontSize type=int, initial=0, invalidate=Style
 minimumLogicalFontSize type=int, initial=0, invalidate=Style
 defaultFontSize type=int, initial=0, invalidate=Style
 defaultFixedFontSize type=int, initial=0, invalidate=Style
 
 editingBehaviorType type=EditingBehaviorType, initial=editingBehaviorTypeForPlatform()
 
 caretBrowsingEnabled initial=false
 localStorageEnabled initial=false
 allowUniversalAccessFromFileURLs initial=true
@@ skipping 222 matching lines @@
 
 # Clients that execute script should call ScriptController::canExecuteScripts()
 # instead of this function. ScriptController::canExecuteScripts() checks the
 # HTML sandbox, plug-in sandboxing, and other important details.
 scriptEnabled initial=false, invalidate=ScriptEnable
 
 # Compensates for poor text legibility on mobile devices. This value is
 # multiplied by the font scale factor when performing text autosizing of
 # websites that do not set an explicit viewport description.
 deviceScaleAdjustment type=double, initial=1.0, invalidate=TextAutosizing