Prevent download in sandbox

Implements the feature while keeping it disabled. Will follow the
Intent to Implement/Ship process to enable it.

Bug: 539938
Change-Id: I83ef89691e5977717cdcf2c9a121177e5de52ab1
Reviewed-on: https://chromium-review.googlesource.com/c/1336493
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613137}
8 files changed