Add MDB comments to recent kNoPendingIO changes

Bug: skia:
Change-Id: I93de3aea986fe64fe536fdf6db8b075c101082b2
Reviewed-on: https://skia-review.googlesource.com/54461
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed