[web-animations-1] Inconsistent default document timeline in Animatable interface

[hiikezoe]

In animate() method section;

if timeline member of options is missing, be the default document timeline of the node document of the element on which this method was called

In KeyframeAnimationOptions dictionary member section;

If missing, the default document timeline of the Document associated with the Window that is the current global object is used

I believe the former is correct and that's what Firefox and Chrome do currently.
@birtles @majido

[birtles]

Good catch. I think the text in the animate() method description actually overrides the description in KeyframeAnimationOptions (since it says what to do when that member is missing) so we should probably just drop the "If missing..." sentence from the description of KeyframeAnimationOptions.timeline.