Simplify StyleResolver::resolveKeyframes()

This change adds Keyframe::cloneWithOffset(), which allows us to simplify
StyleResolver::resolveKeyframes() and avoid repeated calls to
CSSAnimations::isAnimatableProperty().

Also re-order some blocks of code for readability and add some ASSERTs.

BUG=240653
R=alancutter

Review URL: https://chromiumcodereview.appspot.com/23948005

git-svn-id: svn://svn.chromium.org/blink/trunk@157817 bbb929c8-8fbe-4397-9dbb-9b2b20218538