This patch also:
* Removes some now-unnecessary code from nsFlexContainerFrame, which was for jumping from wrapped-placeholders to their out-of-flow frames (for DOM comparisons). This code is now unnecessary because placeholders won't be wrapped anymore.
* Updates some reftests with abspos content to match the updated spec's expectations, with one marked as "fails" for the time being (until bug 1269046 implements css box alignment, which we need to render that test correctly).
MozReview-Commit-ID: 8canWfXk6Kf
