Check for unused variables in GN.

A check for unused variables was missing for some helper functions.
The main thing that this affects is the "config" function, where
people were confused when setting deps or configs didn't work the
way they expected (because they were unused).


Review URL:

Cr-Mirrored-Commit: a425c0d7d5acb3f1a47eef0639e436ff60522ba5
1 file changed