Fix handling of multiple keyframes at same offset

third_party/WebKit/Source/core/animation/InterpolationEffect.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/animation/InterpolationEffect.h"
 
 namespace blink {
 
 void InterpolationEffect::getActiveInterpolations(double fraction, double iterationDuration, Vector<RefPtr<Interpolation>>& result) const
 {
     size_t existingSize = result.size();
     size_t resultIndex = 0;
 
     for (const auto& record : m_interpolations) {
         if (fraction >= record.m_applyFrom && fraction < record.m_applyTo) {
             RefPtr<Interpolation> interpolation = record.m_interpolation;
-            double localFraction = (fraction - record.m_start) / (record.m_end - record.m_start);
+            double recordLength = record.m_end - record.m_start;
+            double localFraction = recordLength ? (fraction - record.m_start) / recordLength : 0.0;
             if (record.m_easing)
                 localFraction = record.m_easing->evaluate(localFraction, accuracyForDuration(iterationDuration));
             interpolation->interpolate(0, localFraction);
             if (resultIndex < existingSize)
                 result[resultIndex++] = interpolation;
             else
                 result.append(interpolation);
         }
     }
     if (resultIndex < existingSize)
         result.shrink(resultIndex);
 }
 
 void InterpolationEffect::addInterpolationsFromKeyframes(PropertyHandle property, const Keyframe::PropertySpecificKeyframe& keyframeA, const Keyframe::PropertySpecificKeyframe& keyframeB, double applyFrom, double applyTo)
 {
     addInterpolation(keyframeA.createInterpolation(property, keyframeB), &keyframeA.easing(), keyframeA.offset(), keyframeB.offset(), applyFrom, applyTo);
 }
 
 } // namespace blink