Rollup merge of servo#17000 - upsuper:webkit-media-feature, r=emilio

Add -webkit-prefix for serialization when necessary

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components/style/gecko/media_queries.rs

@@ -128,6 +128,10 @@ impl ToCss for Expression {
         where W: fmt::Write,
     {
         dest.write_str("(")?;
+
+        if (self.feature.mReqFlags & nsMediaFeature_RequirementFlags::eHasWebkitPrefix as u8) != 0 {
+            dest.write_str("-webkit-")?;
+        }
         match self.range {
             nsMediaExpression_Range::eMin => dest.write_str("min-")?,
             nsMediaExpression_Range::eMax => dest.write_str("max-")?,