We might want UNUSED_ASSERTION as a special case but the unused trace
variable macro was being abused in places.

BUG=angleproject:1290

Change-Id: I5f9252b3ff0484d2e859b9fcf68d5d060b28f1fc
Reviewed-on: https://chromium-review.googlesource.com/324290Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>