Currently the descriptors are moved from the old map to the new map,
which is unsafe for the concurrent marker. This patch removes the map
mutation.

Change-Id: I3f7ce455c7344148a122c7443cf32a4eef0307be
Reviewed-on: https://chromium-review.googlesource.com/535480
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46131}