Adds first step to allowing multithreaded contexts: letting multiple
threads use ANGLE as long as only one thread has a current context at
a time.

Bug: angleproject:4724
Bug: angleproject:4725
Change-Id: I4f606bdb15386cff9e3cb84d4120781e24e15fe4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2269864Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>