Ignore auto z-index when determining IsReplacedNormalFlowStacking.

<foreignObject> is the only element which can be replaced normal flow stacking,
and this element cannot be z-indexed.

Bug: 862635

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ia82a7c648da1d407b38330b7daa280d2baf39d49
Reviewed-on: https://chromium-review.googlesource.com/1137351
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575743}
2 files changed