Always mark content downloaded by devtools delegate as potentially dangerous

Bug: 805445
Change-Id: I7051f519205e178db57e23320ab979f8fa9ce38b
Reviewed-on: https://chromium-review.googlesource.com/894782
Commit-Queue: David Vallet <dvallet@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533215}