Instead of hard-cording the root scroll frame id, get the value from
WebRender. This was previously hard-coded to 0, so when WebRender
switched to using 1 for the root scroll frame id, the positioning of
sticky frames were broken in subtle ways. This happened because they
were being parented to a root reference frame (which now uses the 0 id)
instead of the root scroll frame.
MozReview-Commit-ID: 66ArgKHGpWE
--HG--
extra : rebase_source : ff6937bf7fc8d4472eb074d0466c8dcd6fba54a8
Martin Robinson