Overlap Map Stack Crash

Crash after hitting below bottom of overlap stack: https://code.google.com/p/chromium/issues/detail?id=285979

The crashing scenario happened as follows:

 - force-compositing-mode is disabled. As a result, compositing mode is
   disabled until a compositing trigger.

 - overflow-scroll as the compositing trigger, because it does not enable
   compositing mode before computeCompositingRequirements. Other triggers
   happen during CompositedLayerMapping incremental update entry points.
   (Note: CompositedLayerMapping was formerly known as LayerBacking.)

 - The above two requirements are necessary to create the scenario where
   the root layer does not create an overlap context.

 - The overflow-scroll element has a negative z-index which causes the
   parent layer to be composited, too.

 - As a result, subsequent children layers need to add themselves to the
   overlap map, even if they are not composited.

 - The problem, however, is that we never created an overlap context for
   these negative z-index non-composited layers. When the layers try to add
   themselves to the overlap, they try to access below the bottom of the
   stack, and crash.