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
Now that Declaration implements nsIStyleRule, we don't need the memory
overhead of storing a StyleRule object for style attributes.
We also need to change this prior to patch 5, because the changes in
patch 5 that will allow rules to change (but declarations not) would
otherwise break due to style attribute object merging done by
nsAttrValue::ParseStyleAttribute.
