Bug 926728 - Only pass the first continuation or or-split-sibling of the sticky element to StickyScrollContainer::PositionContinuation. r=dbaron, r=corey

- RestyleManager::RecomputePosition now calls
  StickyScrollContainer::PositionContinuation with the first continuation
  or ib-split-sibling of the passed frame, as PositionContinuations will
  update all continuations at once, and expects the first continuation
  to be passed in.
Bug 926728 - Add crashtest to ensure that assertion is not fired when recalculating the position of a sticky with ib-split siblings. r=corey

- Added crashtest to ensure that the "Should be starting from the first
  continuation" assertion is not fired when recalculating the position of a
  sticky with ib-split siblings.