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/nsDocument.cpp

@@ -12446,3 +12446,16 @@ nsIDocument::GetSelection(ErrorResult& aRv)
 
   return nsGlobalWindow::Cast(window)->GetSelection(aRv);
 }
+
+bool
+nsIDocument::ModuleScriptsEnabled()
+{
+  static bool sEnabledForContent = false;
+  static bool sCachedPref = false;
+  if (!sCachedPref) {
+    sCachedPref = true;
+    Preferences::AddBoolVarCache(&sEnabledForContent, "dom.moduleScripts.enabled", false);
+  }
+
+  return nsContentUtils::IsChromeDoc(this) || sEnabledForContent;
+}