Without this patch, the scrollId for display items inside a fixed-pos
item end as the ASR of the item. In the case of fixed-pos items that are
inside iframes, the ASR is the outer document's root scrollframe. This
means that e.g. wheel-scrolling while over a fixed-pos item inside an
iframe ends up scrolling the outer document's root scrollframe instead
of the iframe's root scrollframe.

In the non-WR codepath, there some APZ machinery that walks up in the
HitTestingTreeNode tree from the hit result, looking to see if that node
has a fixed-pos ancestor, and if so, uses the fixed-pos item's target
APZ as the real hit result. This machinery doesn't exist in WR, because
we don't use the HitTestingTreeNode tree for hit-testing in APZ.
Instead, we need to make sure that the item tag for those display items
already has the appropriate scrollid set.

This patch accomplishes this by introducing a new RAII class that is
pushed into the wr::DisplayListBuilder while we are building display
items inside a nsDisplayFixedPosition, and allows the desired scroll id to
be set on the hit-testing display items.

This behaviour is exercised by test_group_wheelevents, which can now be
enabled with this fix.

MozReview-Commit-ID: L2erPVzJeql

--HG--
extra : rebase_source : 1db630513cb1dc16d4e38649812e81f62c8da99c