[scroll-animations] Certain tests expect animations with an unattached named timeline to hold time at zero

[nmoucht]

There are several tests that expect animations with named timelines that are unattached (ie animation-timeline: none, etc) to hold their time at zero. The spec says for the case where a named timeline isn't found, "Otherwise the animation is not associated with a timeline", which implies that the animation should have a null timeline and the animation should not have a resolved time (and shouldn't show up in getAnimations() either). Tests that I have found that expect this behavior:

• scroll-animations/css/animation-timeline-none.html
• scroll-animations/css/animation-timeline-deferred.html (second two subtests)
• scroll-animations/css/animation-timeline-named-scroll-progress-timeline.tentative.html
• scroll-animations/css/animation-timeline-ignored.tentative.html (last subtest)
• scroll-animations/ css/scroll-timeline-dynamic.tentative.html

[nmoucht]

CC @kevers-google, @andruud and @flackr.