Prompt user before openning a download from extension

Downloads api allows an extension to open a download with a user gesture.
However, the current user gesture check is passed from the renderer,
and it can be easily compromised.
This CL adds a prompt dialog to notify user before opening a download.

BUG=666824

Change-Id: I6ec9709fa183f9784db323603f356dc6aa9ea6e1
Reviewed-on: https://chromium-review.googlesource.com/1069320
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564250}
10 files changed