This solves a race condition using the global context.
Sadly we can't use an unprotected variable safely because
it can be written/read from multiple threads doing MakeCurrent.

Has about a 2 ns regression when tested in Linux/Release per
call. "null" benchmark test went from 27 -> 29 ns/iteration.

Fixes a TSAN warning that popped up in angle_end2end_tests.

Test: DrawCallPerfBenchmark.Run/vulkan_null
Test: EGLContextASANTest.DestroyContextInUse/ES3_Vulkan
Bug: b/168744561
Change-Id: Ic56f3faae81c1087b942a3cfc0e011b9ab439e0f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419641Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>