Add some DCHECKs before serializing recordings into Source proto

All sources are expected to be created with some URL and a valid
 (= nonzero) source id.

Once in a blue moon (query and exact count in go/paste/5840277425094656)
we'd get Sources with no URL or invalid source id on the server-side and
causing problems with analysis queries.

Bug: 1274876, b/186441918
Change-Id: I4d51b46c9b46913485e1200af8ec576cbf18b664
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3307902
Commit-Queue: Yue Ru Sun <yrsun@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#948205}