Fix offset of composeLayerRegion

composeLayerRect() pays attention to the offset of the layer, but
composeLayerRegion() ignores it. This patch makes the behavior of
the region case match the behavior of the rect case.

The difference will only be shown by tests which do not position
the saved layer at the origin.

BUG=20288561
R=ccraik@google.com,djsollen@google.com

Change-Id: Ie22737a8614c508cbffe39aa4c55f7d5434277aa