Make the GrSmallPathAtlasMgr a flush-time object

This will allow the small path renderer to be used w/ DDLs.

This is broken out of the omnibus CL:

https://skia-review.googlesource.com/c/skia/+/307776 (Split the small path renderer into record-time and flush-time pieces)

Bug: 1108408
Change-Id: I64cc6ff677d0aead7cf2f097c0e7fbb15b49d49d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/309304
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed