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This animates the new extensions container when extensions are added/removed from the toolbar.

There will be a follow up that moved the animating layout to toolbar icon container view so that animating layout will also be used for the avatar button.

Bug: 985792
Change-Id: Ia0717e982825ddaa794ad495b20a3ef32bf87060
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1756983
Commit-Queue: Caroline Rising <corising@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697439}