Introduces a phase for marking wrappers in the atomic pause. The phase
is opportunistic in a sense that it may no catch all wrappers. E.g.
there may be wrappers discovered only after processing ephemerons.

Bug: chromium:843903
Change-Id: Ic55fdcb37e3ff44156e4eca4c1c95919fc3fcd1d
Reviewed-on: https://chromium-review.googlesource.com/1166835Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54962}