Don't dispatch beforeunload events for downloads

BUG=812293
TBR=jochen@chromium.org

(cherry picked from commit cd05cee6edd1301f63d44306ff120820b6a4cfde)

Change-Id: I1375ecbdef13ea43ac350cdae6e550267166fadc
Reviewed-on: https://chromium-review.googlesource.com/934204
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#539097}
Reviewed-on: https://chromium-review.googlesource.com/941446
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#623}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
4 files changed