Definitely tricky for classes like SkNWayCanvas, where the caller (today)
need not pay attention to ownership of the canvases it gave the NWay
(after this CL, the caller *must* managed ownership)
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4441
DOCS_PREVIEW= https://skia.org/?cl=4441
Change-Id: Ib1ac07a3cdf0686d78e7aaa4735d45cc90bea081
Reviewed-on: https://skia-review.googlesource.com/4441
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Robert Phillips <robertphillips@google.com>