Adjust fragment state blend factor and write mask validation

[shrekshao]

Issue: https://crbug.com/dawn/2272

Validation in chrome impl needs adjustment: https://dawn-review.googlesource.com/c/dawn/+/164922

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

[kainino0x]

LGTM

[kainino0x]

Sorry, I just reread the spec and I think I misinterpreted this the other day.
IIUC actually the only case where writeMask=0 makes it valid is when the fragment shader has no output to that location. If it has output to that location, then the writeMask isn't checked by the validation in the spec.
https://gpuweb.github.io/gpuweb/#abstract-opdef-validating-gpufragmentstate

I do believe we could loosen the validation in the way that you've done here, but it would require a spec change.

[shrekshao]

I think you are right. The "Otherwise..." is at the same level as

"If get the entry point(FRAGMENT, descriptor) has a shader stage output value output with location attribute equal to index:"

[kainino0x]

I do believe we could loosen the validation in the way that you've done here, but it would require a spec change.

For reference the spec issue was gpuweb/gpuweb#2013