Fix sending multiple DLP reporting events for video capture (warn mode)

Fix for warning mode.

Extend existing browser tests to ensure that exactly one warning event
and up to one warning proceeded event are sent for each video capture
and screenshot.

Bug: 1279347
Change-Id: I6b8e21cd2870c10f6d3e1c360c9c1dd961c222e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3374482


Reviewed-by: Aya Elsayed <ayaelattar@chromium.org>
Reviewed-by: Aida Zolic <aidazolic@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Jan Kopański <jkopanski@google.com>
Cr-Commit-Position: refs/heads/main@{#962172}