It would depend on who had higher speed.
If the Pokemon who used Ion Deluge was faster, then it would go first and the Greninja would change its type. So, in this scenario, its type would be Normal.
If Greninja was faster, it would change its type and then get it changed by Ion Deluge. So, in this scenario, its type would be Electric.
tl;dr If Greninja goes first it's type would be Electric. If it goes after Ion Deluge its type would be Normal.
Source: Knowledge of the move and ability