By default the SPACE key scrolls the page down by full height of browser view. With this script, pressing SHIFT+SPACE will scroll half of the view height. Page scroll by a quarter view height can be done using either LEFTSHIFT+RIGHTSHIFT+SPACE or SHIFT+CAPSLOCK+SPACE (configurable via variable).
Notes:
Older Chrome version (e.g. v43) is not yet capable of differentiating between left and right SHIFT key. i.e. it only recognizes one type of SHIFT key.
Avoid using LEFTSHIFT+RIGHTSHIFT+SPACE for QuarterScrollKey for now, since all web browsers don't properly generate keyup event when both SHIFT keys are held then released.
