Downloads : Fixed an issue of opening incorrect download file

When one download overwrites another completed download, calling download.open in the old download causes the new download to open, which could be dangerous and undesirable. In this CL, we are trying to avoid this by blocking the opening of the old download.

TBR=shaktisahu@chromium.org

(cherry picked from commit a8d6ae61d266d8bc44c3dd2d08bda32db701e359)

Bug: 793620
Change-Id: Ic948175756700ad7c08489c3cc347330daedb6f8
Reviewed-on: https://chromium-review.googlesource.com/826477
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#525810}
Reviewed-on: https://chromium-review.googlesource.com/849195
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/branch-heads/3282@{#403}
Cr-Branched-From: 5fdc0fab22ce7efd32532ee989b223fa12f8171e-refs/heads/master@{#520840}
9 files changed