Traverse layers in update order

bug:8540150

Layers now require traversal in update order, as it will be child
first, then parent for layer-in-layer

Fixes issue with deferred layer playback not flushing in order, and
thus child not painting before parent

Also fixes DisplayList to only be cleared after flush in deferred list

Change-Id: I2f284d00079cdb20798aeef6a1c94e823940db40