Allow using certain special keys in Browser.setGlobalKeyboardShortcut.

Required two changes to JB:
1. Use keydown instead of keypress to attach handlers. (see:
   http://stackoverflow.com/a/8932965)
2. Maintain another table to judge if default action should be prevented.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
Delete annotation when Delete / Backspace key is pressed.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>