Auto merge of #17306 - mantaroh:paced, r=birtles

Return zero when computing distance of Option with both value is none.

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This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1371480

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There are tests for these changes, a test case will be landed in reftests in https://bugzilla.mozilla.org/show_bug.cgi?id=1371480

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components/style/properties/helpers/animated_properties.mako.rs

@@ -853,6 +853,7 @@ impl <T> Animatable for Option<T>
             (&Some(ref this), &Some(ref other)) => {
                 this.compute_squared_distance(other)
             },
+            (&None, &None) => Ok(0.0),
             _ => Err(()),
         }
     }