Disable parallel ephemeron handling if single-threaded flag is set.

Bug: chromium:844008
Change-Id: I96d3daae4a239a5326c6d3394f620697bad1780a
Reviewed-on: https://chromium-review.googlesource.com/1102335Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53761}