Running with ASAN there are several use after free issues because a
eglDestroyContext destroys the context right away even though it's in
use in other thread(s). Adding reference count to context so that it's
not destroyed until all users are done using it.

Bug: b/162609728
Change-Id: I00b24b53d760e38ff61dd9ce652a49b1f32f0cd2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2336447
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>