Fix cases of variable shadowing in /gm/.

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

Change-Id: I905459bf0bbaa205da7dc59e1910e345bed2af51
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/438538
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
13 files changed