Guard against data race with MemoryProgramCache. 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/+/2415182 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>