[VL] Add histogram to track addition of 404 VisitRows

The histogram will be used to monitor the proportion of VisitRow
additions that are for 404 visits, after enabling the saving of 404
visits to the History DB.

The histogram is emitted via macro rather than the corresponding
function because AddPageVisit() is called in a loop for redirects, and
tools/metrics/histograms/README.md says: "when recording metrics on the
critical path (e.g. called in a loop or logged multiple times per
second), use the macros in histogram_macros.h instead [of the histogram
functions]."

This CL is part of a larger project to make links `:visited` after a 404
visit. Project links:
- Google-internal project doc: go/stop-filtering-history-by-status-code
- Issue tree / work status: https://crbug.com/427225439/dependencies

Fixed: 445899720
Change-Id: If0eae711cd3d67e0afe0e9c9f9c8141aaac6ba0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6976394
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Commit-Queue: Svend Larsen <svend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1521560}