Linux: use new xdg-settings "check" feature to determine whether we are the default browser.

BUG=17093
TEST=in GNOME, let Firefox set itself as the default after Chrome has set itself; Chrome should then detect that it is no longer the default

Review URL: http://codereview.chromium.org/155889

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@21260 0039d316-1c4b-4281-b951-d872f2087c98