Since we record slots for weak list fields, there is no need to iterate over
them after evacuation. We just need to update the roots for which we didn't
record slots.

This speeds up the evacuate_update_pointers_weak phase by ~2.5x.

BUG=

Review URL: https://codereview.chromium.org/1772803002

Cr-Commit-Position: refs/heads/master@{#34555}