This patch just adds an optional codepath that isn't taken yet, so it shouldn't
affect our behavior. (The next patch in the series will make use of this new
codepath.)
Note: the large code-comment that this patch adds is taken mostly-verbatim from
some nsFlexContainerFrame.cpp code. (The original copy will be removed by the
next patch in this series, when we switch to take advantage of this new
mechanism.)
MozReview-Commit-ID: 9pkJ346rrXg
