Avoid SkImage ref churn in short lived SkBitmapProvider

SkBitmapProvider is always stack-allocated and tightly-scoped.  It
should be safe to store a SkImage rawptr instead of a ref object.

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2424813002

Review-Url: https://codereview.chromium.org/2424813002
1 file changed