Groups and implicit references are no longer relevant at this point.

Also add tests that fail if the first or second round of ephemeral
marking is omitted

BUG=none
R=mstarzinger@chromium.org,hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/761343004

Cr-Commit-Position: refs/heads/master@{#25606}