Clarify that the GrDirectContext uniquely holds the text atlas manager

Bug: 1108408
Change-Id: Iad0b634a8a0d6dac64e3571a32e755d124401f66
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/309077
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed