move makeAtlasTextOp to SubRun

When SubRun is sub classable, makeAtlasTextOp will be part of
the virtual API.

Change-Id: Ib316bca200de38c5aba64e0bef5fa43966e6f13a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301983
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
5 files changed