Convert evict callback from function to interface

This allows me to find uses of the eviction registratoin
easier in my IDE.

Change-Id: I127911f769d90716f6c8bb69d71b2255786aec21
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265981
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Herb Derby <herb@google.com>
8 files changed