Revert "Change SkCanvas to *not* inherit from SkRefCnt"

Revert "Change SkCanvas to *not* inherit from SkRefCnt"

This reverts commit 824075071885b6b741c141cbe2134d8345d34589.

Reason for revert: Breaking WebView (chromium:663959)

Original change's description:
> 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>
>

TBR=djsollen@google.com,mtklein@google.com,halcanary@google.com,robertphillips@google.com,fmalita@chromium.org,fmalita@google.com,reed@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I5e3b3e876b7d2c09833cf841801321033b6b968b
Reviewed-on: https://skia-review.googlesource.com/4687
Commit-Queue: Heather Miller <hcm@google.com>
Reviewed-by: Heather Miller <hcm@google.com>