Instantiate clip atlases directly

Calls assign() directly on clip atlas proxies instead of using the
scary, unprotected back-pointer from a callback function.

Bug: chromium:928984
Change-Id: I253fc0f3280b97b6af4f1fde2d0094d58dc5d125
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/387296
Reviewed-by: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
5 files changed