Script streaming: Add an option to make the main thread block (wait for parsing)

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 271 matching lines @@
 # (crbug.com/398485).
 # This value is set to false if the platform does not support fullscreen
 # for non-media elements. When set to true all the requests to enter fullscreen
 # on non-media elements will return an error.
 disallowFullscreenForNonMediaElements initial=false
 
 # V8 supports different types of caching. Used by V8 bindings.
 v8CacheOptions type=V8CacheOptions, initial=V8CacheOptionsOff
 
 v8ScriptStreamingEnabled initial=false
+v8ScriptStreamingBlocking initial=false
 
 # 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
 
 accessibilityEnabled initial=false, invalidate=AccessibilityState
 inlineTextBoxAccessibilityEnabled initial=false