Reintroducing parse_a_sizes along with tests

[paavininanda]

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #__ (github issue number if applicable).

• There are tests for these changes

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/test.rs, components/script/dom/bindings/trace.rs, components/script/dom/htmlimageelement.rs
• @KiChjang: components/script/test.rs, components/script/dom/bindings/trace.rs, components/script/dom/htmlimageelement.rs
• @emilio: components/style/values/specified/source_size_list.rs

[paavininanda]

r? @jdm

[emilio]

Also note that this is not needed, SourceSizeList::evaluate already handles this correctly.

[emilio]

I think I'd prefer to remove the unit tests and make the SourceSizeList and SourceSize fields private again, given there's a few tests that check all this in WPT: https://wpt.fyi/results/html/semantics/embedded-content/the-img-element/sizes

[jdm]

I would prefer to retain the unit tests for the time being until we finish implementing all the pieces that are required in order for the WPT tests to actually work in Servo.

[emilio]

That means that when I change SourceSizeList in Gecko I need to update all the tests, which is cumbersome given this is tested already there... :/

[jdm]

Ok, that's a reasonable argument against these changes. @paavininanda Let's undo the changes that were made to support re-adding the unit tests, and remove the unit tests. This PR can just focus on adding the parse_a_sizes_attribute function without using it anywhere.

[jdm]

Ok, that's a reasonable argument against these changes. @paavininanda Let's undo the changes that were made to support re-adding the unit tests, and remove the unit tests. This PR can just focus on adding the parse_a_sizes_attribute function without using it anywhere.

[jdm]

Since this is part of #21181 which makes use of it, let's just wait for that PR to merge.