Issue #618 - (async) Implement async attribute for inline module scripts.

This commit does several things:
- Moves the pref check from ScriptLoader to ns[I]Document so it can be called on
the document.
- Changes the atrribute freezing function to a better name that takes the
document as a parameter.
- Sets the proper async/defer attributes on HTML script elements based on
keywords and whether they are module scripts or not.

dom/script/ScriptElement.cpp

@@ -128,11 +128,11 @@ ScriptElement::MaybeProcessScript()
     return false;
   }
 
-  FreezeUriAsyncDefer();
+  nsIDocument* ownerDoc = cont->OwnerDoc();
+  FreezeExecutionAttrs(ownerDoc);
 
   mAlreadyStarted = true;
 
-  nsIDocument* ownerDoc = cont->OwnerDoc();
   nsCOMPtr<nsIParser> parser = ((nsIScriptElement*) this)->GetCreatorParser();
   if (parser) {
     nsCOMPtr<nsIContentSink> sink = parser->GetContentSink();