Bug 1363481 - Add the old attribute value as a parameter to Element::AfterSetAttr r=bz

In order to facilitate the movement of code with side-effects called by Element::SetAttr to Element::BeforeSetAttr and Element::AfterSetAttr, Element::AfterSetAttr should have access to the old value of the attribute. This includes information about whether there was previously a value set or not.

Accomplishing this involved passing an additional argument through functions that find and change the old attribute value in order to ensure that we can differentiate between an empty old value and an absent old value (attribute was not set).

Note that while I tried to ensure that accurate values (and their absence) are reported to Element::AfterSetAttr, I largely ignored SVG. While the old value reported for SVG values should be however accurate the value already being reported to SetAttrAndNotify was, SVG elements do not currently report unset values properly because they will never pass a null pointer to SetAttrAndNotify.

MozReview-Commit-ID: K1mha8CNFZP

dom/html/HTMLInputElement.cpp

@@ -1398,7 +1398,8 @@ HTMLInputElement::BeforeSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
 
 nsresult
 HTMLInputElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
-                               const nsAttrValue* aValue, bool aNotify)
+                               const nsAttrValue* aValue,
+                               const nsAttrValue* aOldValue, bool aNotify)
 {
   if (aNameSpaceID == kNameSpaceID_None) {
     //
@@ -1521,7 +1522,8 @@ HTMLInputElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
   }
 
   return nsGenericHTMLFormElementWithState::AfterSetAttr(aNameSpaceID, aName,
-                                                         aValue, aNotify);
+                                                         aValue, aOldValue,
+                                                         aNotify);
 }
 
 void