Revert "Reland "Remove use of integers for atlas indexing""

This reverts commit 3ae2cd5ef8906947ee9189299f6f08af07bea8c2.

Reason for revert: Odd issues with fontcache-mt on Vulkan.

Original change's description:
> 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>

TBR=jvanverth@google.com,bsalomon@google.com

Change-Id: I82fb5073d11b52e35b6f731415e2a87f62bc8edb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/171780
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed