|Shell::externalized_contents_| is guarded by |Shell::workers_mutex_|,
but wasn't being acquired when the serialize WriteValue call failed.

Bug: v8:8034
Change-Id: Idd0448e9f44d6b26c17987405d5d7394449e8bb3
Reviewed-on: https://chromium-review.googlesource.com/1170316Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55060}