The Mac now supports native combo box widgets so it no longer will crash in GetProperty
with a null mWidget field.
Modified nsComboboxControlFrame::HandleEvents. MouseExit from the combox rect now
initiates mouse capture on the drop-down list.
Modified nsCSSFrameConstructor::ConstructSelectFrame to initialize the drop-down list
for frame-based comboboxes as hidden.
Added a nsIListControlFrame::CaptureMouseEvents method.
Implemented nsListControlFrame::CaptureMouseEvents.
