Create notification_content_detection subdir under chrome/browser/

With more chromium changes coming related to notification content
detection and warnings on suspicious notifications, there should be a
separate subdirectory under chrome/browser/safe_browsing/. This will
allow us to create more files as needed in a more organized way.

Change-Id: I5f38282d8d13a03b78d35f0a7e1d6591d2b61e52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6390916
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Sarah Krakowiak <skrakowi@chromium.org>
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1438083}