Adjust DomSnapshot to properly handle stacking contexts.

While the goal and documentation for the IsStackingContext states that
it denotes nodes that start a new stacking context, using IsStacked() to
determine this ends up also marks nodes that are absolutely positioned.

R=pfeldman@chromium.org

Change-Id: I99831acefe92a28b18c9184e769a57eaeada3773
Reviewed-on: https://chromium-review.googlesource.com/1222268
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Jandro Alderman <aalderman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591373}
3 files changed