This patch adds an overload to nsIContentIterator::Init which accepts
RangeBoundary objects, and modifies the codepath to avoid using the offset of
the start or end nodes to construct the nsIContentIterator.
MozReview-Commit-ID: 5ZqKeiUunoN
nsIContentIterator::Init() takes nsRange but it's too expensive for some users.
So, there should be another Init() which can be specified a range in DOM tree
with 2 pairs of nsINode* and uint32_t.
MozReview-Commit-ID: 6JXic0KOM2d
