Auto merge of #14663 - ctjhoa:moz-orient, r=Manishearth

Stylo: implement -moz-orient

<!-- Please describe your changes on the following line: -->
https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-orient
![2016-12-21-213132_1342x870_scrot](https://cloud.githubusercontent.com/assets/1716173/21405334/c6c49cb2-c7c5-11e6-85d7-e10f71c331ef.png)

---
<!-- 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
- [X] These changes fix #14198 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it's a style addition

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components/style/properties/longhand/box.mako.rs

@@ -1559,3 +1559,11 @@ ${helpers.predefined_type("-moz-binding", "UrlOrNone", "Either::Second(None_)",
                           products="gecko",
                           animatable="False",
                           disable_when_testing="True")}
+
+// Non-standard: https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-orient
+${helpers.single_keyword("-moz-orient",
+                          "inline block horizontal vertical",
+                          products="gecko",
+                          gecko_ffi_name="mOrient",
+                          gecko_enum_prefix="StyleOrient",
+                          animatable=False)}