WICG/attribution-reporting-api#1097

Although doing so is currently unnecessary for business logic, we
persist the value in the DB to ensure that we can retrieve it in the
future if needed. This also allows us to show it in the Attribution
Internals UI, which can aid in debugging.

We avoid a DB migration by treating absence of the corresponding proto
field as equivalent to the browser's maximum value. A DB version bump is
not needed, as prior to this change, the value was effectively a
constant used only when a source was newly stored, not after, so there
is no risk of a browser downgrade causing misinterpretation.

OBSOLETE_HISTOGRAM[Conversions.SourceRegistrationError9]=Replaced by Conversions.SourceRegistrationError10

Bug: 1500333
Change-Id: I61646ba8edf67946aa04ea7e5739a3963921c273
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5010674
Reviewed-by: Nan Lin <linnan@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1231376}