Rename GrAtlasTextContext -> GrTextContext

Change-Id: I309b39425afc9b45095241eeb299096bc426afed
Reviewed-on: https://skia-review.googlesource.com/130029
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Herb Derby <herb@google.com>
18 files changed