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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components/style/gecko/wrapper.rs

@@ -70,6 +70,16 @@ pub struct GeckoDeclarationBlock {
     pub immutable: AtomicBool,
 }
 
+impl PartialEq for GeckoDeclarationBlock {
+    fn eq(&self, other: &GeckoDeclarationBlock) -> bool {
+        match (&self.declarations, &other.declarations) {
+            (&None, &None) => true,
+            (&Some(ref s), &Some(ref other)) => *s.read() == *other.read(),
+            _ => false,
+        }
+    }
+}
+
 unsafe impl HasFFI for GeckoDeclarationBlock {
     type FFIType = bindings::ServoDeclarationBlock;
 }