Fix DCHECK failure when Manifest fetch has an Access Control error.

Fix DCHECK failure when Manifest fetch has an Access Control error.

This is also updating the test that wasn't able to catch this error
because of unfortunate timing issues.

BUG=474228
Committed: https://crrev.com/cd67622827724269db39c2ccb63d26e63bd41868
Cr-Commit-Position: refs/heads/master@{#325810}

[mlamouri (slow - plz ping), 2015-04-19 09:54:38]

mlamouri@chromium.org changed reviewers:
+ mkwst@chromium.org

[mlamouri (slow - plz ping), 2015-04-19 09:54:39]

Mike PTAL. This is a M43 Dev blocker.

[Mike West, 2015-04-19 12:20:15]

LGTM.

[mlamouri (slow - plz ping), 2015-04-20 08:47:27]

The CQ bit was checked by mlamouri@chromium.org

[commit-bot: I haz the power, 2015-04-20 08:48:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1089873003/1

[commit-bot: I haz the power, 2015-04-20 08:59:01]

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

[commit-bot: I haz the power, 2015-04-20 08:59:02]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[mlamouri (slow - plz ping), 2015-04-20 09:01:24]

mlamouri@chromium.org changed reviewers:
+ jochen@chromium.org

[mlamouri (slow - plz ping), 2015-04-20 09:01:24]

+jochen@, for some reasons, I thought mkwst@ was owing these files...

[jochen (gone - plz use gerrit), 2015-04-20 09:03:25]

lgtm

[mlamouri (slow - plz ping), 2015-04-20 09:05:28]

The CQ bit was checked by mlamouri@chromium.org

[commit-bot: I haz the power, 2015-04-20 09:06:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1089873003/1

[commit-bot: I haz the power, 2015-04-20 09:12:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-20 09:12:52]

Patchset 1 (id:??) landed as
https://crrev.com/cd67622827724269db39c2ccb63d26e63bd41868
Cr-Commit-Position: refs/heads/master@{#325810}