Check for unused variables in GN.

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).

BUG=392026
R=dpranke@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=284509

[brettw, 2014-07-18 00:20:40]

This also depends on WebKit patch https://codereview.chromium.org/403653002/

The yasm code was pretty suspicious. I believe this matches the GYP build (the
makefile was only referenced in one place). I wrote a note to double-check this
change with Albert when he comes back from vacation.

[Dirk Pranke, 2014-07-21 20:14:19]

rubberstamp lgtm :).

[brettw, 2014-07-21 21:48:03]

Committed patchset #1 manually as r284509 (presubmit successful).