It turns out part 10 was actually needed, but Bug 1449838 had a significantly
better version of what that code does that was easy enough to backport to what
we have. As far as I can tell, this passes the relevant reftests now.
This implements flexbox layout for the gap properties without the refactoring work performed on `nsFlexContainerFrame`.
Partially based on https://bugzilla.mozilla.org/show_bug.cgi?id=1398483 and https://bugzilla.mozilla.org/show_bug.cgi?id=1454822
This excludes the second part of bug 1454822, the width caching implementation, because it is out of scope and currently causes unstable layout with `writing-mode: vertical-lr` (see bug 1709937).
Since these are just interpreted comments, there's 0 impact on actual code.
This removes all lines that match /* vim: set(.*)tw=80: */ with S&R -- there are
a few others scattered around which will be removed manually in a second part.
