Prevent parent window scroll in modal mode using overflow hidden

This appears to be the least evil way to achieve this, as scroll
events are not cancelable.

Remove previous technique which only worked for keystrokes.

See http://stackoverflow.com/questions/1459676

Change-Id: Ib0d120676d54b92fdc6fd483088c24d45438feae
Merge "Prevent parent window scroll in modal mode using overflow hidden"