Fix cases of variable shadowing in src/core/.

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: Ifb985c1baa5811c458454016d0d05fb04baa1c56
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/435721
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed