Revert 228032 "Prevent extension side-enabling."

> Prevent extension side-enabling.
> 
> Review URL: https://codereview.chromium.org/26714003

TBR=gab@chromium.org

Review URL: https://codereview.chromium.org/27035004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228188 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228188