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
The web platform tests changes are just a cherrypick of
https://github.com/w3c/web-platform-tests/pull/2926 so I don't have to add
failure annotations until the next test uplift.
I've audited our uses of nsIFormControl, and this patch looks to me like it
preserves existing behavior in all but the following cases:
1) nsXBLPrototypeHandler::DispatchXBLCommand, the case of scrolling when space
is pressed while something inside a <label> is focused. We used to not scroll
in this situation; I think this is a bug, so I'm changing that behavior to
scroll instead.
2) In Accessible::RelationByType for the RelationType::DEFAULT_BUTTON case,
when mContent is a <label> we used to return its form's default submit element.
Now we will just return Relation().
