Fix cases of variable shadowing in /bench/.

If we manage to fix all the existing cases of variable shadowing, we
could enable -Wshadow.

Change-Id: I91187f35a33293914958fba1c0b3b6adf5364e19
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/438318
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Herb Derby <herb@google.com>
6 files changed