GM for SkOverdrawCanvas

Change-Id: Ice39bca25df0255448dce726e5c0d86dce889a60
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205940
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>