don't rely on canvas being reference counted

BUG=skia:

Change-Id: Ibb9feeabd305fa53dcba69c7a383ed8efa7df3bc
Reviewed-on: https://skia-review.googlesource.com/5264
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed