Remove deprecated SkPaintFilterCanvas::onFilter.

No longer used by any clients. The new onFilter is made pure virtual to
enforce that it is implemented and the existing fallback forwarding
implementation is removed.

Change-Id: I8ce522db78d28a1e83779fbfa6979e6e16a0cee5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208507
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
2 files changed