Bug 1115111 - Fix calculation of obscured regions. r=

The code did not correctly handle a scenario where the layer tree is like so:
Root layer
- Layer A
- Layer B
  - Layer C
In this case the hit region of layer C should be used to obscure layer A, but
it was not doing so; only layer B's hit region was being used.
Bug 1115111 - Carry obscured regions across process boundaries with event regions. r=