Bug 1468294 - Double f64::EPSILON for calculation error. r=birtles

It's possible that both this_weight and other_weght have calculation errors
which are approximately equal to f64::EPSILON.

MozReview-Commit-ID: 8OddG9rI3qd

--HG--
extra : rebase_source : 9c22d17dcfb8efea7276864502344dc4981c358a

dom/animation/test/crashtests/1468294-1.html

@@ -0,0 +1,7 @@
+<script>
+addEventListener("DOMContentLoaded", () => {
+  document.documentElement.animate([{ "transform": "matrix(2,1,1,5,2,8)" }],
+                                    { duration: 1000,
+                                      easing: "cubic-bezier(1,-15,.6,4)" });
+})
+</script>

dom/animation/test/crashtests/crashtests.list

@@ -40,3 +40,4 @@ pref(dom.animations-api.core.enabled,true) load 1393605-1.html
 load 1400022-1.html
 pref(dom.animations-api.core.enabled,true) load 1401809.html
 pref(dom.animations-api.core.enabled,true) load 1411318-1.html
+load 1468294-1.html

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

@@ -1739,7 +1739,9 @@ impl Animate for Quaternion {
 
         let (this_weight, other_weight) = procedure.weights();
         debug_assert!(
-            (this_weight + other_weight - 1.0f64).abs() <= f64::EPSILON ||
+            // Doule EPSILON since both this_weight and other_weght have calculation errors
+            // which are approximately equal to EPSILON.
+            (this_weight + other_weight - 1.0f64).abs() <= f64::EPSILON * 2.0 ||
             other_weight == 1.0f64 || other_weight == 0.0f64,
             "animate should only be used for interpolating or accumulating transforms"
         );