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.

parser/html/nsHtml5TreeOperation.cpp

@@ -1076,7 +1076,7 @@ nsHtml5TreeOperation::Perform(nsHtml5TreeOpExecutor* aBuilder,
       nsCOMPtr<nsIScriptElement> sele = do_QueryInterface(node);
       NS_ASSERTION(sele, "Node didn't QI to script.");
       sele->SetScriptLineNumber(mFour.integer);
-      sele->FreezeUriAsyncDefer();
+      sele->FreezeExecutionAttrs(node->OwnerDoc());
       return NS_OK;
     }
     case eTreeOpSvgLoad: {