Auto merge of #12648 - heycam:store-extra-data, r=Manishearth

Store UrlExtraData in {specified,computed}::Image::Url.

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This stores a `UrlExtraData` object in `specified::Image::Url` and `computed::Image::Url`, so that geckolib can use the base/principal to create Gecko `ImageValue`s.  (I'll do that in the followup to support background-image; want to get this in first.)

r? @Manishearth

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