modernize some picture internal types

More sk_sp, unique_ptr, less ownership convention.
A little bit of warmup for the work in the bug.

Bug: skia:9796
Change-Id: If6ce37883e4175617d377e74c8cc44353ea47e16
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265443
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
7 files changed