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/base/nsIDocument.h

@@ -2851,6 +2851,8 @@ public:
     return mIsWebComponentsEnabled;
   }
 
+  bool ModuleScriptsEnabled();
+
 protected:
   bool GetUseCounter(mozilla::UseCounter aUseCounter)
   {