Currently, pop_back() doesn't update size. Fix that.

BUG=v8:7489

Change-Id: If11d2ddcaae3223289c168c1e6e1ad94d6775516
Reviewed-on: https://chromium-review.googlesource.com/934456Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51517}