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When we're using a button to trigger tab switches, we don't want the
description or icon hinting at switching, since the main suggestion
won't switch in this state.

In addition to these tiny changes, it adds a new icon for the button,
per the UI spec.

Bug: 780835
Change-Id: I01d0c913292852e6ecaf569c1feecccbebd0646d
Reviewed-on: https://chromium-review.googlesource.com/943831
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540338}