Fix -Wunused-but-set-variable warnings

Change-Id: I5a044983e3de84901f29bddca5503167edc8fcee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/535436
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
6 files changed