Animation: Remove ASSERT_NOT_REACHED from LengthStyleInterpolation

Source/core/animation/LengthStyleInterpolation.cpp

Index: Source/core/animation/LengthStyleInterpolation.cpp
diff --git a/Source/core/animation/LengthStyleInterpolation.cpp b/Source/core/animation/LengthStyleInterpolation.cpp
index 225341dfc0876cfe39b9750c05c7999889598e30..5af721d428e40c28138b806c7c6d57bb2c4e925d 100644
--- a/Source/core/animation/LengthStyleInterpolation.cpp
+++ b/Source/core/animation/LengthStyleInterpolation.cpp
@@ -95,7 +95,8 @@ PassRefPtrWillBeRawPtr<CSSPrimitiveValue> LengthStyleInterpolation::interpolable
 
     switch (unitTypeCount) {
     case 0:
-        ASSERT_NOT_REACHED();
+        // TODO: this case should never be reached. This issue should be fixed once we have multiple interpolators.
+        return CSSPrimitiveValue::create(0, CSSPrimitiveValue::CSS_PX);
     case 1:
         for (size_t i = 0; i < CSSPrimitiveValue::LengthUnitTypeCount; i++) {
             const InterpolableNumber *subValueType = toInterpolableNumber(listOfTypes->get(i));