Use StringKeyframes to make CSS Animations responsive to style changes.

Use StringKeyframes to make CSS Animations responsive to style changes.

When a animated property is absent in the 0% or 100% keyframe, a neutral
keyframe is created which takes the value of the computed style.  When
the computed style changes, the spec requires that the neutral keyframe
is updated accordingly to match the new style.

This patch switches from AnimatableValueKeyframe over to StringKeyframe
in CSS animations. The only place that still uses AnimatableValueKeyframe
is transitions.

This patch also introduces behaviour changes for interpolating
SVG stroke-width, opacity and shape-image-transform. The new behaviours
of these interpolations in CSS animations match that of web animations.

When there is a non-animation style change, neutral keyframes are updated.
Firefox handles this correctly, but IE11 doesn't.

BUG=361948

[shend, 2015-02-13 04:01:16]

shend@chromium.org changed reviewers:
+ alancutter@chromium.org, dstockwell@chromium.org, shans@chromium.org,
timloh@chromium.org

[shend, 2015-02-13 04:01:17]

This is the patch that actually switches CSS animations to use responsive
behaviour.