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This CL sets the kTabSearch flag to be enabled by default on
ChromeOS and leaves it disabled by default on the remaining
desktop platforms (Linux, Mac, Windows).

This is part of the required launch cleanup for the ChromeOS
Tab Search experiment.

Launch config CL:
https://critique-ng.corp.google.com/cl/347894919

Bug: 1116182
Change-Id: I4de20453851b095a62c099a02c8dcebfb26ca00c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596029
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Yuheng Huang <yuhengh@chromium.org>
Reviewed-by: Roman Arora <romanarora@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838959}