We introduce a new Display Global Mutex in egl::Display that
can guard access to gActiveCompilers in Compiler.cpp.

Was detected by running MultithreadingTest against TSAN.

Bug: b/168744561
Change-Id: I97866b60a173f60899cd0406fe0f71000035c0cf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2415181
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>