Make privet notification only start if the preference to enable them is true

Make privet notifications start/stop beased on the preference to enable/disable
them.

BUG=23621055
R=vitalybuka@chromium.org

Review URL: https://codereview.chromium.org/23621055

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224618 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224618