Bug 1829225 - Make BeforeSetAttr take the parsed nsAttrValue. r=smaug

ParseAttribute ideally would be const (see bug 1829138), but the SVG and
SMIL code is rather messy. Still, now that BeforeSetAttr can't really
fail, swapping the order of ParseAttribute and BeforeSetAttr shouldn't
really change behavior.

Sorry for the extra `virtual` keyword removal and such. I had to do this
one by hand unlike the dependent bugs, and I went a bit drive-by, lmk if
you want me to split those changes.

Differential Revision: https://phabricator.services.mozilla.com/D176086

dom/html/HTMLFormElement.cpp

@@ -180,8 +180,7 @@ NS_IMPL_ELEMENT_CLONE(HTMLFormElement)
 nsIHTMLCollection* HTMLFormElement::Elements() { return mControls; }
 
 void HTMLFormElement::BeforeSetAttr(int32_t aNamespaceID, nsAtom* aName,
-                                    const nsAttrValueOrString* aValue,
-                                    bool aNotify) {
+                                    const nsAttrValue* aValue, bool aNotify) {
   if (aNamespaceID == kNameSpaceID_None) {
     if (aName == nsGkAtoms::action || aName == nsGkAtoms::target) {
       // Don't forget we've notified the password manager already if the