Fix Wunused-but-set-variable warnings

Change-Id: I0e093fd35b11e9a765ef9c09f3b6346086ff66bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/435983
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
11 files changed