Add shared mutex to frontend API entrypoints that is only enabled when
ANGLE is being used with a shared context.

Bug: angleproject:2464
Change-Id: I0d918e37d9579dccd013dc88f563bed7de7ee55f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1685712Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>