Handle missing or invalid Keyframe offsets.

The keyframes are normalized using the algorithm from
section 6.16.4 'Normalizing the list of keyframes' of the Web Animations draft specification.

BUG=327564

Review URL: https://codereview.chromium.org/133003004

git-svn-id: svn://svn.chromium.org/blink/trunk@165599 bbb929c8-8fbe-4397-9dbb-9b2b20218538