dawn: use alpha version of BlendFactor for wgpu::BlendState::alpha

Bug: b/283485842
Change-Id: Ia6ec23af78744e8a18c4d77e3240b7669760cf61
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/700637
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
1 file changed