Calling quit() from d8 will call exit(), which will run static destructors. If
context_mutex_ is statically allocated, pthread_mutex_destroy will be called.

When running d8 in "isolates" mode, another thread may be running. If it calls
CreateEvaluationContext, it will lock the context_mutex_. If the mutex is
destroyed while it is locked, it will return an error.

This CL changes the Mutex to a LazyMutex, which will leak instead of being
destroyed.

BUG=v8:4279
R=jarin@chromium.org
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1240553003

Cr-Commit-Position: refs/heads/master@{#29709}