Enable GL by default if GPU enabled and no backend selected.

Change-Id: I0e4b1d56edc364217b753886caeea9c158d8be28
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215644
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed