Fixing focus problems with the combobox

Fixing some combobox focus related issue.
The combobox now needs to subclass the HWND to report focus changes to the FocusManager (as the FocusManager is not subclassing anymore).
Made NativeControlWin always subclass for that reason, which led to getting rid of NativeControl::NotifyOnKeyDown()

Previously reviewed in: http://codereview.chromium.org/151002

BUG=http://crbug.com/6900
TEST=Open the option dialog. Focus a comboxbox. Deactive then reactivate the
dialog. The focus should be on the combobox. Click on the combobox to bring-up
the list. Press ESC, this should close the list popup (not the dialog). Make
sure all other components (buttons, checkboxes, radio-buttons, text-field) work
as expected.