Enable justify-items:legacy propagation for Servo

[nicoburns]

This value was added for implementing the legacy <center> and align attribute. Unlike other justify-items it inherits. Gecko, Servo, and Blitz all use the older text-align: -moz-left solution for that. But Gecko implements this property at style-level only. This PR simply implements that style adjustment for Servo builds as well.

Makes a couple of css-align tests pass.

Servo PR: servo/servo#48806

[Loirooriol]

Yes, it was added for this purpose, but it's not suitable for it. See w3c/csswg-drafts#11463

legacy needs to be removed, if allowed by web compat.

[nicoburns]

Ah, maybe we can just close this then. It affects like 3 subtests. They just happen to be 3 that would take Blitz to 100% coverage of a particular WPT subsuite which was the main reason I was pursuing this.

[Loirooriol]

It may still make sense if it's not going to be removed soon. What are the tests, is Servo affected too?

[nicoburns]

It's the ones Blitz doesn't pass here: https://blitz.is/wpt/css/css-align/default-alignment
Servo is also affected.

[Loirooriol]

I guess as long as legacy is supported, it's better to do it as the spec currently says, so feel free to reopen this.

[Loirooriol]

Please prepare the Servo PR with the new test expectations