change SkCanvasStack to take ownership of its subcanvases

Inspired by https://bugs.chromium.org/p/chromium/issues/detail?id=663959

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4799

Change-Id: I69f7ac73386bb7ca96778e2fec4cb2757b982a52
Reviewed-on: https://skia-review.googlesource.com/4799
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
4 files changed