Per bug 1299753 comment 20, SetFlags() is designed for flag propagation
to continuations during block reflow. To avoid misuse, I expand
SetFlags() directly in the only reasonable caller nsBlackFrame::Init(),
and replace other usages by AddStateBits().
MozReview-Commit-ID: GsbE2Z0Rps1
--HG--
extra : rebase_source : 72a64e9218870d638f67d1b586f533cd7d16c491