Reenable overview mode by default (revert of 225406).

BUG=297636
R=jamescook@chromium.org

Review URL: https://codereview.chromium.org/25070002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225764 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225764