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.

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-Commit-Position: refs/heads/master@{#525810}
DownloadFileFactory* DownloadManagerImpl::GetDownloadFileFactoryForTesting() {
  return file_factory_.get();
}

DownloadFileFactory* DownloadManagerImpl::GetDownloadFileFactoryForTesting() {
  return file_factory_.get();
}
