DawnMTLWindowContext assumed default device
DawnMTLWindowContext assumed default device

Dawn has removed the MTLDevice query that we were using. Fixes viewer
on Mac.

Change-Id: I09fe74b362c3201653094062032052c7da6c3b2a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/571496
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>
Reviewed-by: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>
Auto-Submit: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Commit-Queue: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>

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