Prevent creating background pages for non-incognito profile in Guest mode

BUG=329498
TEST=manually

Review URL: https://codereview.chromium.org/101413011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242271 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242271