Fix bug in SkMipmapCache::AddAndRef factory selection.

Discovered via ClangTidy `readability-static-accessed-through-instance`.

Change-Id: I646e3293853e65b5cc2419c8687c035aab4b669a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/308659
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed