Fix sending multiple DLP reporting events for video capture

Fix for report mode.

Add a browser test which ensures that exactly one event is sent for each
video capture.

Bug: 1279347
Change-Id: I33faf8b7b6972449a2b34af81e38647a135e177f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3365046
Reviewed-by: Aya Elsayed <ayaelattar@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Aida Zolic <aidazolic@chromium.org>
Commit-Queue: Jan Kopański <jkopanski@google.com>
Cr-Commit-Position: refs/heads/main@{#960923}