Add metrics for chrome.contentSettings extension usage

This CL adds metrics for how often chrome-extension:// origins are
used to set content settings via the chrome.contentSettings API, to
see if this can be deprecated.

Bug: 40086360
Change-Id: Ide2b22a6e628fd8bebbd227032e5e4c7264c1c1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5814282
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Emily Stark <estark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1348438}