Remove TextBlob's empty mailbox in favor of delegate

The empty mailbox is a bit awkward with a buffet build because
the client would have to select the "empty" implementation
or incur link errors from SkTextBlob.cpp

This has the gpu backend register a callback function to be
invoked when the textblob is being destroyed.

Change-Id: I90ce506961a5ced41f68c5519977228268b836f7
Bug: b/40045064
Reviewed-by: Jim Van Verth <>
Commit-Queue: Kevin Lubick <>
Reviewed-by: Brian Osman <>
12 files changed