Fix GraphiteDawnTestContext to allow creation of different backends.

Bug: b/315834710

Change-Id: I93fa2a544b2cb4729e59e7115175850428c8a97e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/788437
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
2 files changed