ANGLE will make a context current during deletion, so ANGLE can release
related resources, but for an external context, the API user should make
sure the native context is current during deleting the ANGLE external
context, so ANGLE doesn't need to make the context current.

Bug: angleproject:5509
Change-Id: Ia4cee4e3965e9e9e811e3f989f6f6d72bd940f41
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2640593
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>