After moving unreachable weak global handles only process harmony collections

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}
Perform smaller marking steps incrementally in idle notification.

BUG=

Review URL: https://codereview.chromium.org/767233003

Cr-Commit-Position: refs/heads/master@{#25607}