Enable GL by default if GPU enabled and no backend selected.
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 <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Commit-Queue: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>

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