Make the persistent cache utility functions operate on SkData

This avoids boilerplate at every call-site (and more are being added
soon) to set up the reader/writer objects.

Change-Id: Idc632932b5a9be8d3164d249abab7dccbb905ca3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207704
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed