Web Animations API: Constructing an Animation from a partial keyframe throws a JS exception

Web Animations API: Constructing an Animation from a partial keyframe throws a JS exception

Depends on https://codereview.chromium.org/203463009/

Previously associated with issue https://codereview.chromium.org/190763007/

Previously, if an animation was constructed with a partial keyframe (a keyframe list with no
keyframe at offset zero or no keyframe at offset one for any of the properties it declares)
the renderer would crash.

This change detects partial keyframes and throws a NotSupportedError in the JavaScript.

BUG=

[rjwright, 2014-03-19 07:42:53]

ptal