Web Animations: Simplify keyframe input handling.

(1) Throw exceptions rather than failing silently for out-of-range offsets.
(2) Always throw exceptions for unsorted offsets, rather than only when
    null offsets are present.
(3) Remove duplicate (and silently-failing) error detection from
    KeyframeEffectModel.

See https://web-animations.etherpad.mozilla.org/ep/pad/view/ro.92487irtSfEow/rev.20441 for minutes where we decided to make this change.

Review URL: https://codereview.chromium.org/407393002

git-svn-id: svn://svn.chromium.org/blink/trunk@178753 bbb929c8-8fbe-4397-9dbb-9b2b20218538