Fix benches that aren't really timing what we want

Remove full clears from benches
Fix unbounded canvas modification in GM benches

Bug: skia:
Change-Id: Ie3d67282714a7b4e980aec399056c0e9df330993
Reviewed-on: https://skia-review.googlesource.com/86040
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed