Apply some clang-tidy suggestions to graphite code

Change-Id: I6c68e8a69175f5cc9033d265e44252e5fb389e0c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/894839
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
6 files changed