[tools/viewer] fix backend type string in viewer
[tools/viewer] fix backend type string in viewer

Add "Dawn (Graphite)" backend name, also change kBackendTypeStrings
string array to a method, it is less problematic.

Bug: skia:13804
Change-Id: I73ea817241deedf126056e249a7d5b51a8bc6f60
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/594576
Commit-Queue: Arman Uguray <b18dd57f0c81bc1fc439e303e71d2866b09f3bd2@google.com>
Auto-Submit: Peng Huang <18764e26bc5bdc5f80483a2fbf46c7fcc384c153@chromium.org>
Reviewed-by: Arman Uguray <b18dd57f0c81bc1fc439e303e71d2866b09f3bd2@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia