[omnibox] Use "sentence case" for suggestion group headers.

In order to align with the latest UX requirements, this CL updates the
relevant code to switch over from ALL CAPS to "sentence case" for the
text that's used to render suggestion group headers in the Omnibox
popup.

BEFORE: https://screenshot.googleplex.com/9ep2VGwgXVVVBW3
AFTER:  https://screenshot.googleplex.com/BiKpTWkv6VT49t4

Bug: 406855215
Change-Id: Idf00860380be629bffda3dd14bc21b4c5b595e0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6456527
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Khalid Peer <khalidpeer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1447817}