Tab Search: Disable Tab Search button in incognito mode

Bug: 1099917
Change-Id: Ie74de530af3fdb7aa87ed14ac14aba47c7fd5b4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432677


Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811078}