Simplify subrun atlas generation tracking

The atlas generation's tracking was in two places. Gather
all into regenerate.

Gather more of the bulk use tracking into regenerate, and
cleanup some of the bulk use comments.

Change-Id: I87727b087d4605de9fa4684f4662fa49dc424863
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268164
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed