Ed, punting this to you if that's OK.

Fantastic! I love animations, especially since I learned some cool techniques from reviewing your CLs. 

Do we have specs or mocks or descriptions of these animations?

CL in-flight.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9d8520be330f34fc964c4d73077356e1ab9f3af5

commit 9d8520be330f34fc964c4d73077356e1ab9f3af5
Author: edchin <edchin@chromium.org>
Date: Thu Mar 22 14:40:19 2018

[ios] Animate tab grid empty state when closing last tab

This CL animates the appearance of the empty state prompt as the last
tab closes. The animation scales and fades in.

Bug:  804558 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I1449560d6be676030048e11cf32db21123c37e5f
Reviewed-on: https://chromium-review.googlesource.com/974576
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545070}
[modify] https://crrev.com/9d8520be330f34fc964c4d73077356e1ab9f3af5/ios/chrome/browser/ui/tab_grid/grid/grid_view_controller.mm

Beautiful animation~~ looking so good. =)

The NextAction date has arrived: 2018-03-28