Revert "Skip over nested handlesOwnChildren nodes in getRelativeOffset"

Breaks ve-mw tests, because:
* getRelativeOffset() can now apparently return -1, but
  doesn't document this
* getRelativeRange() uses relative offsets to construct
  ranges, so it returns ranges like (-1, -1)
* ve-mw tests for getRelativeRange() rightly fail
  when they receive ranges like (-1, -1)

This reverts commit 9b2102f796d28d9e938cbda7600694863d0805c8.

Change-Id: I1f2f0f497d2f87d1064d76806f9ea206caddcdc1
Merge "Revert "Skip over nested handlesOwnChildren nodes in getRelativeOffset""