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

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

When an animation is sampled at a fractionalTime of 1,
KeyframeAnimation::fetchIntervalEndpointsForProperty() selects the last
keyframe for both the 'previous' and 'next' keyframes to be used for blending.
This results in a scale of infinity and AnimationBase::fractionalTime()
resturns a value of -nan.

When -nan is passed to the default timing function, we hit the ASSERT() in
UnitBezier::solveCurveX(). If a linear timing function is used, the calculated
property value is -nan, and we hit the ASSERT() in the CSSPrimitiveValue
constructor.

This change fixes the problem by ignoring the scale in
AnimationBase::fractionalTime() in this case. This is safe because the
fractionalTime is always zero once the offset has been accounted for, so the
correct result is zero.

R=dstockwell

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=157564

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

lgtm

https://codereview.chromium.org/23451031/diff/4001/Source/core/page/animation...
File Source/core/page/animation/AnimationBase.cpp (right):

https://codereview.chromium.org/23451031/diff/4001/Source/core/page/animation...
Source/core/page/animation/AnimationBase.cpp:466: if (offset)
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 00:11:51]

https://codereview.chromium.org/23451031/diff/4001/Source/core/page/animation...
File Source/core/page/animation/AnimationBase.cpp (right):

https://codereview.chromium.org/23451031/diff/4001/Source/core/page/animation...
Source/core/page/animation/AnimationBase.cpp:466: if (offset)
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.

[commit-bot: I haz the power, 2013-09-11 00:20:41]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/steveblock@chromium.org/23451031/11001

[commit-bot: I haz the power, 2013-09-11 02:17:38]

Change committed as 157564