Suppress file type warnings for local file:// downloads

This adds a special carveout for local file:// downloads which
suppresses DANGEROUS_FILE (i.e. file type) warnings for these downloads.
A file type warning for a local file download is not helpful since the
file is already on disk.

Makes the concept of overriding the FileTypePolicies more explicit.

Bug: 379052288
Change-Id: Ic42d86dc07cbcaae4a49f590a34c89ecc1104e65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6040129
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1388232}