Shim to enforce that canvas virtuals are overridden

Change-Id: Iad70d449bbc43195baefd70d20c41996a8435e6e
Reviewed-on: https://skia-review.googlesource.com/113320
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
15 files changed