Updated to use sentinel GL context even when GL backend is not built.
This reverts commit 1171d314ef.
Change-Id: Ia94bbe4865ddd4e898446c13886877c539f0eb0b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277976
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>