Make atlases instantiate themselves at flush time

Change-Id: I5ed9794bd78c8e5d999e892f5672b9c590b44285
Reviewed-on: https://skia-review.googlesource.com/106000
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed