Start cleanup of GrTextBlob

* Make a single private section
* Reorganzie private section
* Start making const things const

Change-Id: I88081d02ee6034234d35d15306bffb47ce63ac8c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257688
Commit-Queue: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
2 files changed