WebGPU: Implement blending Constant alpha is not handled yet. It needs to be emualted or added to WebGPU. Bug: angleproject:370690274 Change-Id: I5f8402f74ab08345b2f02c18e2d43fa39b2479a7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6194554 Reviewed-by: Liza Burakova <liza@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>