Revert of Enable Site Isolation for Extensions.

Revert of Enable Site Isolation for Extensions. (patchset #3 id:40001 of https://codereview.chromium.org/2285883002/ )

Reason for revert:
Moving back to Site Isolation for Extensions being off by default, until we fix the remaining functional issues.

Original issue's description:
> Enable Site Isolation for Extensions.
> 
> This CL enables isolated extensions. Removal of the command line flag and
> the field trial will be done in a separate CL.
> 
> BUG=545200
> 
> Committed: https://crrev.com/d8f0aefde00132b06bd97cb17555f2ec89a0c203
> Cr-Commit-Position: refs/heads/master@{#414879}

TBR=alexmos@chromium.org,rdevlin.cronin@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=545200

[nasko, 2016-10-28 18:05:38]

The CQ bit was checked by nasko@chromium.org

[nasko, 2016-10-28 18:05:39]

Created Revert of Enable Site Isolation for Extensions.

[commit-bot: I haz the power, 2016-10-28 18:06:00]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-28 18:09:19]

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

[commit-bot: I haz the power, 2016-10-28 18:09:20]

Try jobs failed on following builders:
  ios-device on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device/builds...)