Initial implementation of progress based ScrollTimelines

This work is the first step in implementing upcoming spec change in:
w3c/csswg-drafts#4862

Converted AnimationTimeline::currentTime() to CSSNumberish
Added AnimationTimeline::duration

Bug: 1140602

Change-Id: I5c9b8b7130b91faae5f172493ba5f55d77b4bdd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476900
Commit-Queue: Jordan Taylor <jortaylo@microsoft.com>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826533}