Fix cross context test to actually use shared GL contexts

It's incredible that this was working on so many devices and GL implementations.

Bug: skia:
Change-Id: I8d43405b1eaea170e28840f17a7088d2f59aa612
Reviewed-on: https://skia-review.googlesource.com/20449
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed