This also happens if user kills browser during a dangerous download, and then resumes it by restarting the browser process.

When resuming a dangerous download after browser crash, we will call DownloadTargetDeterminer to re-determine the download target.

However, DownloadTargetDeterminer will recheck the download url and the referer during resumption, as the safe browsing list might have changed after download starts.

Checking download url and the referer can potentially change the
danger_type back to something unsafe.

That causes the download controller to show the dangerous download infobar when download item updates its observers.

However, since the download item is loaded from history service, there is no valid webcontent for it to show the infobar. This causes the download resumption to silently fail.

qinmin: Can you confirm that this is happening for pretty much all .apk downloads? If so, that's important to track down.

Yes, i can reproduce the issue with following steps:

1. download any apk.
2. while downloading, turn wifi off 
3. kill Chrome
4. turn wifi on
5. restart chrome and wait for download to resume
6. download will fail silently

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d5d9f828c9c9366fa219e6a7aad6eccfdd2dd1fd

commit d5d9f828c9c9366fa219e6a7aad6eccfdd2dd1fd
Author: qinmin <qinmin@chromium.org>
Date: Fri Jan 20 01:55:02 2017

Fix a problem that user validated dangerous download cannot resume

After a dangerous download become user validated, it should be treated as not dangerous.
However, Chrome currently still checks for downloadUrl and referer.
This CL fixes the problem and added a test for the case.

BUG= 682468 

Review-Url: https://codereview.chromium.org/2641063002
Cr-Commit-Position: refs/heads/master@{#444941}

[modify] https://crrev.com/d5d9f828c9c9366fa219e6a7aad6eccfdd2dd1fd/chrome/browser/download/download_target_determiner.cc
[modify] https://crrev.com/d5d9f828c9c9366fa219e6a7aad6eccfdd2dd1fd/chrome/browser/download/download_target_determiner_unittest.cc