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This change makes ExtensionsToolbarContainer invisible by default. It
turns visible when any extension is added and goes invisible if the last
extension is removed.

Bug: chromium:943702
Change-Id: Ia64d8db55d3cd53beff450941d1b7d2cba282ca0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1956186
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737171}