Add flag to disable demo mode.

R=dpolukhin@chromium.org
BUG=343727

Review URL: https://codereview.chromium.org/168853003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252412 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252412