It is undefined behaviour to delete a mutex that is locked by a thread.  To
ensure that the global mutex is not locked, lock/unlock it before deleting it.

BUG=angleproject:2464

Change-Id: I735e924abfa06070f2b4103230be8593649ca340
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1690674
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>