Reland "Remove use of integers for atlas indexing"

This is a reland of 3a8f345cf5689c6e1f22e852acfde4b8b1e5ba42

Original change's description:
> Remove use of integers for atlas indexing
> 
> Bug: skia:
> Change-Id: I7c29e90de6531a35c415f0338e23c176a7293040
> Reviewed-on: https://skia-review.googlesource.com/c/171233
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Jim Van Verth <jvanverth@google.com>

Bug: skia:
Change-Id: Ifb041f74028a119ba410e80cbfaedce34614f90b
Reviewed-on: https://skia-review.googlesource.com/c/171539
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed