Adds ultra-deferred proxies that are instantiated by a user-supplied
callback during flush.
Bug: skia:7190
Change-Id: I75a7ac6dba953c3b0a99febc203a7f4d2f3789fc
Reviewed-on: https://skia-review.googlesource.com/76461
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
