Changes appearance of the NTP for the guest mode.

1. Returns icon of the incognito guy to the tab header.
2. Sets NTP as homepage for the guest mode.
3. Adds the missing separator between the options.

BUG=chromium-os:9534
TEST=manual

Review URL: http://codereview.chromium.org/5314002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@67125 0039d316-1c4b-4281-b951-d872f2087c98