Add a UMA metrics to track preference updates

Adding a metric to track which preference are updated.

An udpate to a preference is triggering an update of the file on disk.
The update is done though the ImportantFileWrite which is suspected to
be too heavy on disk flush and it is affecting Chrome performance.

Bug: 40667025
Change-Id: Ic8a1d58e2e781eb919980a1c4eba873acdce44e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5391881
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Cr-Commit-Position: refs/heads/main@{#1282506}