Auto merge of #16765 - CJKu:bug-1310885-part-1, r=heycam

stylo: Export mozilla::css::ImageValue to stylo.

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This is "Bug 1310885 - Part 0. (gecko) Export mozilla::css::ImageValue to stylo."
We need to land it separately to prevent build break.

Link to the gecko bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1310885.

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