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This CL creates a new file for TabSearchButton browser tests. These
tests run for both the fixed and regular entrypoints.

Existing tests for the TabSearchButton have been moved from the tab
strip into the TabSearchButton browser tests.

Bug: 1099917
Change-Id: I0b1d57826768705316b9bed814dac7af96ce52e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422730
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809376}