Fix the BlockLargeFileTransfer and AllowPasswordProtectedFiles policies

Per the documentation, these should only apply to files selected for
deep scanning. The current implementation, however, applies these
restrictions to all downloads.

Bug: 980777
Change-Id: I62e80d590def15e19d2ba1740356390a757ae628
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849415
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Bettina Dea <bdea@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706300}