Fix overlap testing of negative z-index compositing elements

This patch fixes a bug where negative z-index elements were skipping
checking composited content behind them.

Additionally, more comments were added, and
pushCompositingContainer/popCompositingContainer is renamed to
beginNewOverlapTestingContext/finishCurrentOverlapTestingContenxt.
The push/pop concept is actually misleading because state after
popping is not the same as before pushing.

BUG=264970

Review URL: https://chromiumcodereview.appspot.com/20872002

git-svn-id: svn://svn.chromium.org/blink/trunk@155217 bbb929c8-8fbe-4397-9dbb-9b2b20218538