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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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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components/style/build_gecko.rs

@@ -624,6 +624,7 @@ mod bindings {
             .whitelisted_function("Gecko_.*");
         let structs_types = [
             "mozilla::css::GridTemplateAreasValue",
+            "mozilla::css::ImageValue",
             "mozilla::css::URLValue",
             "mozilla::Side",
             "RawGeckoAnimationPropertySegment",
@@ -764,6 +765,11 @@ mod bindings {
             "RawGeckoServoStyleRuleList",
         ];
         for &ty in structs_types.iter() {
+            // XXX cku: will be removed in Part 2.
+            if ty.starts_with("mozilla::css::ImageValue") {
+                builder = builder
+                    .raw_line("#[allow(unused_imports)]");
+            }
             builder = builder.hide_type(ty)
                 .raw_line(format!("use gecko_bindings::structs::{};", ty));
             // TODO this is hacky, figure out a better way to do it without