Enable dead_belowdot with de layout.

AltGr + j returns dead_belowdot in de layout.
E.g. AltGr + j, s returns U+1E63 with de.
XLookupString can get dead_belowdot but the return value is 0.
I think XLookupString does not failed with return value 0.

BUG=RH#921164

Review URL: https://codereview.appspot.com/7812045