servo: Merge #15720 - Added deep equal operator for animation value (from shinglyu:animation-value); r=Manishearth

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This is the servo side patch for [Bug 1337229](https://bugzilla.mozilla.org/show_bug.cgi?id=1337229)

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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Source-Repo: https://github.com/servo/servo
Source-Revision: 9a438ab3b40bd6ed5e4f8c2ae1fcb90f9ca27175

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : cd7ec826cfe5ec84a37b26dc0a25b3036e81e79f

servo/ports/geckolib/glue.rs

@@ -316,6 +316,16 @@ pub extern "C" fn Servo_AnimationValue_GetTransform(value: RawServoAnimationValu
     }
 }
 
+#[no_mangle]
+pub extern "C" fn Servo_AnimationValue_DeepEqual(this: RawServoAnimationValueBorrowed,
+                                                 other: RawServoAnimationValueBorrowed)
+     -> bool
+{
+    let this_value = AnimationValue::as_arc(&this);
+    let other_value = AnimationValue::as_arc(&other);
+    this_value == other_value
+}
+
 /// Takes a ServoAnimationValues and populates it with the animation values corresponding
 /// to a given property declaration block
 #[no_mangle]