This replaces the linked list of weak cells with a worklist.

TBR=yangguo@chromium.org

BUG=chromium:694255

Change-Id: Ia877e25010ebbec9c05fbbe48cff460a92d3a132
Reviewed-on: https://chromium-review.googlesource.com/587067
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46929}