Reshow the notification after validating dangerous download

Reshow the notification after validating dangerous download

This patch adds the code to reshow the notification after user clicks "Keep" button on dangerous download notification.

This patch also adds the tests of dangerous downloads.

BUG=480489
TEST=browsertest run

Committed: https://crrev.com/9f853ad8b093476f2ec2a7b29f227315a266b780
Cr-Commit-Position: refs/heads/master@{#329821}

[yoshiki, 2015-05-12 17:51:02]

yoshiki@chromium.org changed reviewers:
+ asanka@chromium.org

[yoshiki, 2015-05-12 17:51:13]

Patchset #1 (id:1) has been deleted

[yoshiki, 2015-05-12 17:54:59]

Patchset #1 (id:20001) has been deleted

[yoshiki, 2015-05-12 17:56:47]

Asanka, PTAL. Thanks.

[asanka, 2015-05-14 03:48:01]

lgtm

[yoshiki, 2015-05-14 06:03:53]

The CQ bit was checked by yoshiki@chromium.org

[commit-bot: I haz the power, 2015-05-14 06:04:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1130353004/40001

[commit-bot: I haz the power, 2015-05-14 06:07:44]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-05-14 06:07:46]

Try jobs failed on following builders:
  ios_rel_device_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)

[yoshiki, 2015-05-14 07:42:27]

The CQ bit was checked by yoshiki@chromium.org

[yoshiki, 2015-05-14 07:42:27]

The patchset sent to the CQ was uploaded after l-g-t-m from asanka@chromium.org
Link to the patchset: https://codereview.chromium.org/1130353004/#ps60001
(title: "rebase")

[commit-bot: I haz the power, 2015-05-14 07:42:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1130353004/60001

[commit-bot: I haz the power, 2015-05-14 08:55:59]

Committed patchset #2 (id:60001)

[commit-bot: I haz the power, 2015-05-14 08:56:44]

Patchset 2 (id:??) landed as
https://crrev.com/9f853ad8b093476f2ec2a7b29f227315a266b780
Cr-Commit-Position: refs/heads/master@{#329821}