Change menu item for incognito tab to New Incognito Tab

This CL changes new tab menu item to "New Incognito Tab" for Incognito
mode.

Bug: 1243332
Change-Id: Ibec663e98c472878f98df0ea2b94fcbc8c0f21dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3162196


Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Side YILMAZ <sideyilmaz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#922636}