Add a flag to show a warning dialog for dangerous download

This dialog will replace the original infobar when the flag is enabled

Bug: 1234151
Change-Id: I96a4fd07c62d82807ed8cbfba6d387437da867fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3059761


Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908711}