Add auto purging for SkPicture-related Ganesh resources (esp. layers)

This is intended to lower the bookkeeping burden for the Layer Caching feature. Cached layers are now automatically purged when a picture is deleted.

R=bsalomon@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/408923002
13 files changed