Send <iframe srcdoc> to the threaded parser

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 149 matching lines @@
 touchAdjustmentEnabled initial=true
 
 # A mostly-stable performance optimization. crbug.com/304518 tracks removal.
 compositorTouchHitTesting initial=true
 
 syncXHRInDocumentsEnabled initial=true
 cookieEnabled initial=true
 navigateOnDragDrop initial=true
 DOMPasteAllowed initial=false
 
+// FIXME: Remove this. Unfortunately many webkit_unit_tests
+// depend on parsing being synchronous. crbug.com/366354
 threadedHTMLParser initial=false
 useThreadedHTMLParserForDataURLs initial=false
 
 allowCustomScrollbarInMainFrame initial=true
 webSecurityEnabled initial=true
 
 # Special keyboard navigation mode intented for platforms with no
 # proper mouse or touch support, such as a TV controller with a remote.
 spatialNavigationEnabled initial=false
 
@@ skipping 94 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