Fix cases of variable shadowing in gm/.

These were reviewed last year at http://review.skia.org/312480 but never
landed. Splitting into small chunks to land individually.

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

Change-Id: Ie6a262328315a5726265ef4afede7e0e66337752
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/435718
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
6 files changed