Welcome: Add burst animations to the lines with color

The animation spec outlines an animation that requires the start point
and the end point of each line moving along a cubic-bezier curve. To
accomplish this as close as possible, this CL adds an animation of a
line and its fill. See [1] as reference.

The animations also require JS animations because the spec defines a
delay _after_ each animation, while CSS animations only allow for
delays _before_ each animation or delays on a linear curve.

[1] https://jsfiddle.net/dpj8uoxz/

Bug: 1177471
Change-Id: I47a94ce7d7ee22d062534ef5df23d94db08bd6b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2823410
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#872977}