Only allow downloading in response to real keyboard modifiers

BUG=848531

Change-Id: I97554c8d312243b55647f1376945aee32dbd95bf
Reviewed-on: https://chromium-review.googlesource.com/1082216
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564051}