This assumes that the specification for how position:sticky behaves for
block-in-inline splits matches the specification for position:relative,
in other words, matches
http://www.w3.org/TR/CSS21/visuren.html#anonymous-block-level .
It's also necessary for patch 9b since the new rule for handling of
style change hints is that a style change hint applies to all
continuations and all block-in-inline siblings ("special siblings").
The change in StickyScrollContainer::GetScrollRanges is really the fix
for bug 918994, but adjusted for the change here to use block-in-inline
siblings ("special siblings") in addition to continuations.
