script: Check for render-blocking documents before continuing (#43150)

The spec expects us to check for render blocking (among some other
state) before we start procesing a document. Therefore, add those checks
and also check in the
stylesheet loader that we should only do so when we are allowed to do
so.

Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>

components/script_bindings/webidls/HTMLScriptElement.webidl

@@ -19,6 +19,8 @@ interface HTMLScriptElement : HTMLElement {
            attribute boolean async;
   [CEReactions]
            attribute boolean defer;
+  [SameObject, PutForwards=value]
+           readonly attribute DOMTokenList blocking;
   [CEReactions]
            attribute DOMString? crossOrigin;
   [CEReactions, SetterThrows]