Replace uses of deprecated dawn_native::BackendType

Bug: dawn:824
Change-Id: I261c3329e53de62df83b4cca10aa9e909dd293df
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/492456
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Stephen White <senorblanco@google.com>
Commit-Queue: Stephen White <senorblanco@google.com>
6 files changed