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
Bug: chromium:1087084
Change-Id: Ia6ca48c5fa838b93e49fc9ea259d626029439ca3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2257273
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>