Auto merge of #16665 - longsonr:1347409, r=heycam,emilio

Servo changes for bug 1347409

bug 1347409 is introducing a fallback type for SVG paint servers so that we can distinguish between url(something) and url(something) none and serialise those cases correctly. When we get a url(something) color we need to set the mFallbackType to eStyleSVGFallbackType_Color.

This change has already been reviewed in https://bugzilla.mozilla.org/show_bug.cgi?id=1347409

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because they are covered by tests that will land in gecko as part of the rest of the patch there.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/16665)
<!-- Reviewable:end -->

components/style/properties/gecko.mako.rs

@@ -395,6 +395,7 @@ fn color_to_nscolor_zero_currentcolor(color: Color) -> structs::nscolor {
     pub fn set_${ident}(&mut self, mut v: longhands::${ident}::computed_value::T) {
         use values::computed::SVGPaintKind;
         use self::structs::nsStyleSVGPaintType;
+        use self::structs::nsStyleSVGFallbackType;
 
         let ref mut paint = ${get_gecko_property(gecko_ffi_name)};
         unsafe {
@@ -423,6 +424,7 @@ fn color_to_nscolor_zero_currentcolor(color: Color) -> structs::nscolor {
         }
 
         if let Some(fallback) = fallback {
+            paint.mFallbackType = nsStyleSVGFallbackType::eStyleSVGFallbackType_Color;
             paint.mFallbackColor = color_to_nscolor_zero_currentcolor(fallback);
         }
     }