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This CL updates the TabSearchBubbleView and the TabSearchButton
such that there can only be a single instance of the
Tab Search UI shown at any given time.

This CL also changes the TabSearchButton such that it enters the
active state immediately on bubble creation so that the bubble's
status is communicated to the user while it loads in
asynchronously.

This behavior mimics that in the IconLabelBubbleView.

Bug: 1099917
Change-Id: Ifb5602e55750fd6dd4b834c4dc42835ca08d0f2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354509
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800585}