In bug 1895870 I (somewhat unintentionally) made this code not called at
all.
Instead of scheduling a 30ms timer if reflow was interrupted, we just
schedule a reflow for the next refresh driver tick via
EnsureLayoutFlush(). Which is what would happen anyways before my change
in presence of other changes (like page loading appending content or
what not).
I think thus that the new behavior is more predictable (the 30ms timer
was never quite measured, it was introduced in bug 499447).
So, just simplify the code.
Differential Revision: https://phabricator.services.mozilla.com/D210890
e5d519d1c8