Bug 870628 - [MMS] Text in input box can overflow below the interface.

Fix regression bug https://bugzilla.mozilla.org/show_bug.cgi?id=870628. Main root cause is bottomBar.style.maxHeight is not exist and  bottomBarMaxHeight would be NaN => bottomBar.style.height would never updated because of this and layout busted when input height > bottomBar height. In this patch we ues input height + padding for bottomBar height