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

Source/core/page/animation/AnimationBase.cpp

Index: Source/core/page/animation/AnimationBase.cpp
diff --git a/Source/core/page/animation/AnimationBase.cpp b/Source/core/page/animation/AnimationBase.cpp
index 48bcd2c2d28b9800e21530be6e75507b98b3e8fc..053eb5a9ac3bfcc57c2f078d8b9b825707cba9bc 100644
--- a/Source/core/page/animation/AnimationBase.cpp
+++ b/Source/core/page/animation/AnimationBase.cpp
@@ -463,9 +463,14 @@ double AnimationBase::fractionalTime(double scale, double elapsedTime, double of
         || m_animation->direction() == CSSAnimationData::AnimationDirectionReverse)
         fractionalTime = 1 - fractionalTime;
 
-    if (scale != 1 || offset)
-        fractionalTime = (fractionalTime - offset) * scale;
+    if (offset)

[dstockwell, 2013/09/10 22:19:42]

I had a hard time understanding that the special case here is scale == infinity,
since the conditions don't explain that.

The code looks correct, but there might be a better way to write it. Some ideas:

if (std::isinf(scale)) {
  fractionalTime -= offset;
  ASSERT(fractionalTime == 0);
} else {
  ASSERT(scale >= 0);
  fractionalTime = (fractionalTime - offset) * scale;
}

Or even:
fractionalTime -= offset;
if (std::isinf(scale)) {
  ASSERT(fractionalTime == 0);
} else {
  ASSERT(scale >= 0);
  fractionalTime *= scale;
}

[Steve Block, 2013/09/11 00:11:52]

I guess I don't see scale == infinity as a special-case. We only need to apply
the scale if scale != 1.0 and fractionalTime != 0 (with offset already applied).
In this case, we can assert that the scale is finite. This seems the more
logical way to arrange the if statements, rather than testing scale. I'll add a
comment.

+        fractionalTime -= offset;
+    if (scale != 1.0 && fractionalTime) {
+        ASSERT(scale >= 0 && !std::isinf(scale));
+        fractionalTime *= scale;
+    }
 
+    ASSERT(fractionalTime >= 0 && fractionalTime <= 1);
     return fractionalTime;
 }