code_cache_map_ is used to store the serialized code that could
be consumed in the subsequent runs. This is a std::map which is not
thread safe. So protect this with mutex to avoid any race conditions
when executing in multiple isolates in parallel.

Bug: chromium:783124
Change-Id: Ie58402b8547cf3e83716b8d97d98a298745e487f
Reviewed-on: https://chromium-review.googlesource.com/806334Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49837}