Fix color space conversion test

[greggman]

The issue was makeInPlaceColorConversion was not actually doing the conversion "in place". To find this I wrote a test to test makeInPlaceColorConversion

Note: I removed a maintenance_todo because colorSpace is in the IDL but it's required so it will never be undefined except on old browsers and so TS things comparing it to undefined is an error.

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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 w/ nits

[shaoboyan]

(Not a comment Just for curious).
Would you mind to explain a bit about how this test could detect the behaviour change from = assignment to Object.assign ?

[greggman]

It detects it because makeInPlaceColorConversion, at least by name, is supposed to make an object that is updated "in place"

          const floatColor = { R, G, B, A: 1 };          
          conversionFn(floatColor);
          return [
            floatToU8(floatColor.R),
            floatToU8(floatColor.G),
            floatToU8(floatColor.B),
            floatToU8(floatColor.A),
          ];

This code expects that floatColor is updated. In the assignment (=) version, a new object is created inside the conversion function, That object is then discarded. The function out here is still looking at floatColor with the original unconverted values. When it goes to compare them to the values converted via 2D canvas, they don't match since they're still the unconverted values.

In order to work as it was, this code would need to change to this

          const floatColor = { R, G, B, A: 1 };          
          const convertedColor = conversionFn(floatColor);
          return [
            floatToU8(convertedColor.R),
            floatToU8(convertedColor.G),
            floatToU8(convertedColor.B),
            floatToU8(convertedColor.A),
          ];

I'm happy to change it to this style. I was just going from the momentum given that the function is called makeInPlaceColorConversion