Add chrome://flags entry for AP Download verdicts

This flag will be used for testing out the new download protection
verdicts for Advanced Protection users. Enabling this flag requests the
new verdicts.

Bug: 917190
Change-Id: I5649b7946685c20c5dcf9ec235678d746238b335
Reviewed-on: https://chromium-review.googlesource.com/c/1403981
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621773}
5 files changed