Change SkCanvas to *not* inherit from SkRefCnt

Change SkCanvas to *not* inherit from SkRefCnt

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>