Reland "Add a fieldtrial testing entry for GlicEntrypointVariations."

This is a reland of commit 82567fd05425a41038248a3b076763fb24b9a60e

The original change enabled GlicEntrypointVariations for tests which
among other effects changes the ownership of GlicButton's expansion
animation from the TabStripActionContainer to GlicButton. This reland
fixes a few issues with that:

* make sure to still create the container's animation for the other
  buttons (ButtonOwnsAnimation() should only return true for glic
  button)
* make sure existing end-of-animation logic runs
* in tests that manipulate or check GlicButton animation, use the
  correct animation

Original change's description:
> Add a fieldtrial testing entry for GlicEntrypointVariations.
>
> Change-Id: Icaf9235dd6d07caa35083c8a5437754e1b763b71
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7007313
> Commit-Queue: Ian Wells <iwells@chromium.org>
> Commit-Queue: Sana Akbani <sanaakbani@google.com>
> Auto-Submit: Ian Wells <iwells@chromium.org>
> Reviewed-by: Sana Akbani <sanaakbani@google.com>
> Cr-Commit-Position: refs/heads/main@{#1525127}

Bug: 448624346
Change-Id: Ib846920c2283d361bd0be83f38351982880c1fad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7012507
Commit-Queue: Ian Wells <iwells@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1525895}