Sync web-platform-tests PR 28201 into mozilla-central (this bug is closed when the sync is complete).
Update offsetParent behavior in shadow trees
New behavior for offsetParent in shadow trees was discussed in  and
. This lead to a chromium patch  which changed the behavior. After
 landed, the desired behavior in the discussions in  and 
seemed to have changed. Unfortunately, the author of  was no longer
working on chromium. This new behavior was then added to the spec and
landed in webkit  and firefox , and a WPT was added for it .
This patch implements the new behavior to follow suit with webkit and
firefox based on the WPT in . Unfortunately, there are several tests
which are either internal to chromium or are only passing in chromium
which appear to oppose this new behavior and will have to be updated or
For shadow-dom/offsetParent.html, I verified that firefox and safari
both currently fail the same tests which this patch does.