Expose chrome, devices, and help Chrome URL hosts.

This will list these URLs in chrome://chrome-urls.
They'll also be suggested in omnibox autocomplete.

BUG=297421
R=pkasting@chromium.org
TEST=chrome://chrome, chrome://devices, and chrome://help are all listed in about:chrome-urls and suggested in autocomplete.

Review URL: https://codereview.chromium.org/23441085

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225317 0039d316-1c4b-4281-b951-d872f2087c98