Now that the UI layer is accessible cross-platform, coalesce the DesktopNotificationService layer into one common module.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/343066

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30652 0039d316-1c4b-4281-b951-d872f2087c98