Bug 656197 part 1. Remove the generic attr preparsing mechanism from BeforeSetAttr and just preparse class attributes directly in the one place that needs to do it. r=smaug

This removes the requirement that BeforeSetAttr comes before AttributeWillChange
(which needs the preparsed new value).

MozReview-Commit-ID: 87C6Mjc7ARh

dom/html/HTMLDetailsElement.cpp

@@ -46,7 +46,7 @@ HTMLDetailsElement::GetAttributeChangeHint(const nsIAtom* aAttribute,
 
 nsresult
 HTMLDetailsElement::BeforeSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
-                                  nsAttrValueOrString* aValue, bool aNotify)
+                                  const nsAttrValueOrString* aValue, bool aNotify)
 {
   if (aNameSpaceID == kNameSpaceID_None && aName == nsGkAtoms::open) {
     bool setOpen = aValue != nullptr;