Creates a new class StatsCounterThreadSafe to be used by counters that
can be updated when compiling/decoding etc. are done using workers.

Does this by using a mutex on all opreations.

Also updates the StatsCounterThreadSafe constructor to force counter
initialization, as well as method Reset(). In addition, whenever the
method StatsTable::SetCounterFunction() is called (from the main
thread), it forces counter initialization for all thread safe stats
counters.

BUG=v8:6361

Review-Url: https://codereview.chromium.org/2887193002
Cr-Commit-Position: refs/heads/master@{#45526}