Fix AnimationBase::fractionalTime() to handle a scale of infinity

Source/core/page/animation/KeyframeAnimation.cpp

Index: Source/core/page/animation/KeyframeAnimation.cpp
diff --git a/Source/core/page/animation/KeyframeAnimation.cpp b/Source/core/page/animation/KeyframeAnimation.cpp
index ad1422c7bc79f70ac95f0138afaa8e272164d9b2..5d951422f0c714292f726dc01d5d2d467f43975d 100644
--- a/Source/core/page/animation/KeyframeAnimation.cpp
+++ b/Source/core/page/animation/KeyframeAnimation.cpp
@@ -158,6 +158,8 @@ void KeyframeAnimation::fetchIntervalEndpointsForProperty(CSSPropertyID property
 
     offset = prevKeyframe.key();
     scale = 1.0 / (nextKeyframe.key() - prevKeyframe.key());
+    // A scale of infinity is handled in AnimationBase::fractionalTime().
+    ASSERT(scale >= 0 && (!std::isinf(scale) || prevIndex == nextIndex));
 
     const TimingFunction* timingFunction = 0;
     if (const CSSAnimationData* matchedAnimation = getAnimationFromStyleByName(fromStyle, name()))