move GlyphCacheProc into fontpriv

Bug: skia:2664
Change-Id: I55c84cfae33f8861d66c7a500b940225c1ba8107
Reviewed-on: https://skia-review.googlesource.com/c/170277
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed