[omnibox] Replace IsBrowserUpdateAvailable with IsIncognitoModeAvailable

This CL inhibits the Incognito Pedal when Incognito mode is not
available according to the profile, modeling the check after
a similar function in AppMenuModel, making use of IncognitoModePrefs.

Since the IsBrowserUpdateAvailable method is no longer used, this
CL cleans it up by replacement.

Bug: 1153377
Change-Id: Ie2c68c122e3b185ac996168e776a859c03d2322d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2581813


Commit-Queue: Orin Jaworski <orinj@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835486}