Uses the display global mutex.

Caught with angle_end2end_tests MultithreadingTest and TSAN.

Bug: b/168744561
Change-Id: I5a60346cb5f95ff506dc166604eeae501863a774
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2415182Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>