Add about:flags support for doc.write script blocking.

third_party/WebKit/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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 fetchIncreaseFontPriority initial=false
 fetchIncreaseAsyncScriptPriority initial=false
 # Increases the priorities for CSS, Scripts, Fonts and Images all by one level
 # and parser-blocking scripts and visible images by 2.
 # This is used in conjunction with logic on the Chrome side to raise the threshold
 # of "layout-blocking" resources and provide a boost to resources that are needed
 # as soon as possible for something currently on the screen.
 fetchIncreasePriorities initial=false
 
 # Whether to disallow network fetches for parser blocking scripts in the main
-# frame inserted via document.write.
+# frame inserted via document.write, for users on slow connections.
+disallowFetchForDocWrittenScriptsInMainFrameOnSlowConnections initial=false
+
+# Whether to disallow network fetches for parser blocking scripts in the main
+# frame inserted via document.write, regardless of connection type.
 disallowFetchForDocWrittenScriptsInMainFrame initial=false
 
 # The autoplay gesture override experiment is described in crbug.com/487345 .
 autoplayExperimentMode type=String, initial="off"
 
 # When DevTools is emulating mobile device, frame resize may mean device size has changed,
 # which in turn affects media queries.
 resizeIsDeviceSizeChange initial=false
 
 # Whether the browser process sends gesture events for wheel events.
 # Can be removed when crbug.com/598798 is fixed.
 wheelGesturesEnabled initial=false