Reset current context on Mac GLContext destruction.

Looks like the OS now holds on to the current GL context and doesn't
release it unless explicitly told to.

Bug: skia:9885
Change-Id: Ic7bc6860ec96d88e278682b7f2f67e1ac365d27b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/269369
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Auto-Submit: Jim Van Verth <jvanverth@google.com>
2 files changed