This is pretty much a straight-forward change except for a single thing, the
UpdateInsertionParent calls.
However, I cannot make any sense of them. They go through the inserted children
setting the insertion point, but then ClearInsertionPoints() is called.
ClearInsertionPoints calls XBLChildrenElement::ClearInsertedChildren, which sets
all the insertion points to null anyway.
Thus, I've removed that function completely.
MozReview-Commit-ID: 80daGQfLZrV
--HG--
extra : rebase_source : d52a37a60147ac11794c3cfe1aad5d202e9d2d9f
273bc65bb1