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/svg/SVGScriptElement.cpp

@@ -137,7 +137,7 @@ SVGScriptElement::GetScriptCharset(nsAString& charset)
 }
 
 void
-SVGScriptElement::FreezeUriAsyncDefer()
+SVGScriptElement::FreezeExecutionAttrs(nsIDocument* aOwnerDoc)
 {
   if (mFrozen) {
     return;

dom/svg/SVGScriptElement.h

@@ -44,7 +44,7 @@ public:
   virtual bool GetScriptType(nsAString& type) override;
   virtual void GetScriptText(nsAString& text) override;
   virtual void GetScriptCharset(nsAString& charset) override;
-  virtual void FreezeUriAsyncDefer() override;
+  virtual void FreezeExecutionAttrs(nsIDocument* aOwnerDoc) override;
   virtual CORSMode GetCORSMode() const override;
 
   // ScriptElement