This replaces ad-hoc weakness in normalized map cache with weak cell.

Bug: chromium:694255
Change-Id: I6a12301b2176fe3723b56178a65582cfb412f7d2
Reviewed-on: https://chromium-review.googlesource.com/704834
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48344}