Move class into impl

I think this finishes all public subclasses, so we can proceed next with
moving the virtuals themselves to a private subclass.

Change-Id: I490f3cf6497a6bdcafc1ce756cfdeb32eab18585
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/411239
Reviewed-by: Tyler Denniston <tdenniston@google.com>
Commit-Queue: Mike Reed <reed@google.com>
5 files changed