Add .Then() skeleton and reorder methods

Adding .Then() skeleton to help split up work. .Then() currently just
returns the AnimationBuilder, but the functionality will be added in
future CLs.
Methods reorganized to design doc outline.

Bug: 1236069
Change-Id: I3d800ea64a73f3827cb757a8087fd260644b49cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3068261


Commit-Queue: Elaine Chien <elainechien@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908289}