Mark JSWeakCollections and EphemeronHashTables concurrently to
the application to reduce pause times.

Bug: chromium:844008
Change-Id: I0a8fe592e651b368c5c12846d02d55bbf4058ff2
Reviewed-on: https://chromium-review.googlesource.com/1095035Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53641}