Right now, a XBL <constructor> runs before Custom Elements inside of its
<content> get upgraded. This leads to unexpected behavior where deck.selectedIndex = N
causes selectedIndex to get set as an expando property on the DOM node rather
than running the setter defined by the Custom Element.
Once the Custom Element does finally get upgraded, the selectedIndex getter and
setter don't get attached since there's an expando property with the same name.
This isn't a case we want to have to support from calling code. So this patch fixes
this one case by manually upgrading the element inside the constructor before
anything accesses the node. In Bug 1470242 we are planning to make this happen
behind the scenes so we don't need to do this for every CE inside of <content>.
MozReview-Commit-ID: 3D0QbOOJvDI
909a433f0f