SkPaintFilterCanvas skip-draw support

At the time SkPaintFilterCanvas was introduced as a SkDrawFilter replacement,
no clients were relying on the draw veto logic.  Now Chromium does.

To facilitate migrating off SkDrawFilter, let's augment SkPaintFilterCanvas
with skip-draw semantics.

A side effect of the CL is that now we call the filter virtual even for null
paints.

BUG=skia:4782
R=reed@google.com,robertphillips@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1577933002

Review URL: https://codereview.chromium.org/1577933002
5 files changed