[Maintenance] Feed SkSerialProcs to SkBinaryWriteBuffer

Instead of passing an empty SkSerialProcs struct, create one that encodes images as pngs to SkBinaryWriteBuffer

Change-Id: Icaf529db67f3f46c8e4e454b50b704c1e13a5780
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1279256
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>