Auto merge of #13553 - birtles:animvalues, r=Manishearth

Support creating StyleAnimationValue objects from Servo

These are the servo-side changes for [bug 1302949](https://bugzilla.mozilla.org/show_bug.cgi?id=1302949#c59). @Manishearth has already reviewed them there.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [bug 1302949](https://bugzilla.mozilla.org/show_bug.cgi?id=1302949#c59)
- [X] These changes do not require tests because there are existing tests for this in mozilla-central

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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13553)
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tests/unit/stylo/Cargo.toml

@@ -13,6 +13,7 @@ doctest = false
 
 [dependencies]
 app_units = "0.3"
+cssparser = {version = "0.7"}
 env_logger = "0.3"
 euclid = "0.10.1"
 lazy_static = "0.2"

tests/unit/stylo/lib.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 extern crate app_units;
+extern crate cssparser;
 extern crate env_logger;
 extern crate euclid;
 extern crate geckoservo;