Fix bug in DeallocateGlobalMutex(). Bug: angleproject:6851 Change-Id: Ie5dd32220a38a9b7a9a18ff830a4ee7b7c02b9c3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4381863 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Igor Nazarov <i.nazarov@samsung.com> Reviewed-by: Charlie Lao <cclao@google.com>