Workaround for double tab handling in Chromium

Chromium seems to read the keypress directly before we have a
chance to map it.  So if tab is mapped to something else, don't
send the tab after the timeout.  Otherwise, Chromium will tab
twice.

This fix is hacky, but it works.  Unfortunately, it's useless.
I wanted to use ISO_Left_Tab as Hyper_L when holding and keep
it as tab while tapped.  But mapping ISO_Left_Tab to Hyper_L in
xmodmap causes tab completion in bash to break (using urxvt at
least).