Revert of Restore base::ThreadRestrictions::ScopedAllowIO to see if it fixes linux bot failures.

Revert of Restore base::ThreadRestrictions::ScopedAllowIO to see if it fixes linux bot failures. (https://codereview.chromium.org/274193004/)

Reason for revert:
This patch was landed speculatively due to the suspicion that the removal of the ScopedAllowIO was to blame for http://crbug.com/372017. The cause of that issue was found to lie elsewhere.

Original issue's description:
> Restore base::ThreadRestrictions::ScopedAllowIO to see if it fixes linux bot failures.
> 
> BUG=61827
> NOTRY=true
> NOTREECHECKS=true
> R=asanka@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269405

TBR=mef@chromium.org
BUG=61827
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=271502

[asanka, 2014-05-19 17:17:58]

Created Revert of Restore base::ThreadRestrictions::ScopedAllowIO to see if it
fixes linux bot failures.

[asanka, 2014-05-19 17:18:36]

The CQ bit was checked by asanka@chromium.org

[commit-bot: I haz the power, 2014-05-19 17:19:37]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asanka@chromium.org/292803002/1

[mef, 2014-05-19 18:02:02]

lgtm

[commit-bot: I haz the power, 2014-05-19 19:00:54]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  linux_chromium_chromeos_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-19 20:19:24]

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

[commit-bot: I haz the power, 2014-05-19 20:19:25]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)

[asanka, 2014-05-19 20:58:40]

The CQ bit was checked by asanka@chromium.org

[commit-bot: I haz the power, 2014-05-19 20:59:40]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asanka@chromium.org/292803002/1

[commit-bot: I haz the power, 2014-05-19 22:18:56]

Change committed as 271502