Use navigation for <a download>

We pass the suggested filename along with the request and use that
as a signal that the navigation should result in a download.

The next step will be to ignore this hint for cross origin downloads,
so that only files with a content-disposition will end up being
downloaded.

Bug: 714373,797292
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo;master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I508d7abe1cba9b75d65132b0688984cdebfc6fd4
Reviewed-on: https://chromium-review.googlesource.com/758236
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Jialiu Lin <jialiul@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Min Qin(OOO 12/7-1/10) <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526475}
53 files changed