GitLab

This CL does the following:
1) Move flushing of ephemeron pairs to the start of AdvanceMarking
  (also increase frequency by 2x and make step counters actually
  count steps).
2) Introduce additional (disabled) scopes for diagnostics.
3) Increase frequency of deadline checks when draining worklists (by
   ~8x)
4) Revise worklist draining so that the deadline is checked before
   draining starts.

Bug: 986235
Change-Id: Ie24827a52cbcb497426fd51bc052fb129c417702
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283743
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786229}