DLP: Filter out fake entries in DlpMetadataProvider

Fixes the `TypeError: Value must be an instance of Entry` while fetching
DLP metadata.

R=lucmult@chromium.org

Fixed: 1382371
Change-Id: Ibdbc5f3c87bccba91efdb756c26d00d95aaa5f68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4016391


Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Auto-Submit: Aida Zolic <aidazolic@chromium.org>
Commit-Queue: Aida Zolic <aidazolic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1071514}