Add missing entry to TestExpectations

This one also needed a rebaseline.
virtual/softwarecompositing/z-order/collect-layers-does-not-initialize-pos-z-order-list.html

Unreviewed gardening.

TBR=ojan@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/27088002

git-svn-id: svn://svn.chromium.org/blink/trunk@159492 bbb929c8-8fbe-4397-9dbb-9b2b20218538