After marking ephemerons process the worklist in parallel.

Bug: chromium:844008
Change-Id: Id750c19d0f564675f051cc4f9933d41fa0afb42b
Reviewed-on: https://chromium-review.googlesource.com/1097490
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53694}