[minor] nanobench: use default member initializer

Change-Id: I0324b89d44805b18d5d31a27ec3825633daeb32d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/250659
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Herb Derby <herb@google.com>
1 file changed