[WebGPU] Do not require alphaBlend and colorBlend in GPUColorStateDescriptor

Following WebGPU spec change at https://github.com/gpuweb/gpuweb/issues/370,
alphaBlend and colorBlend are not required anymore in GPUColorStateDescriptor.

Bug: 877147
Change-Id: I74260aaa3c3f662d3a44dcbbd7ac53b7208a2633
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1699678
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#677739}
2 files changed