Add "New incognito window" to the app menu

This CL focuses on UI changes. The functionality will be hooked up in
the next CL.

Bug: 429518328
Change-Id: I802beb160065e3a14f6b394530e2b30f34d96c09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6778233
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Shu Yang <shuyng@google.com>
Cr-Commit-Position: refs/heads/main@{#1491110}