Font metrics returns both internal and external leading to layout. Layout now is responsible for calculating the normalLineHeight. It take consideration of external leading, and compensate if it does not exist. All these are controled by a preference switch. r=rbs, sr=attinasi, git-svn-id: svn://10.0.0.236/trunk@118679 18797224-902f-48f8-a5cc-f745e15eee43