Bug 854740: PopupNotifications doesn't handle showing a dismissed notification after showing a normal one, r=MattN

--HG--
extra : rebase_source : b0da4054ae15276f0b37d951675ff284a54bff3a
Bug 851586, r=bz

--HG--
extra : rebase_source : 4144764238b0f38f1457358b1c9f61bc4d347424