Make TextBlob 'Text' allocators public.

This will allow users to create PDFs with the actual text embedded. This
will allow for correct search and copy operations on the generated PDF.

Since these are now public, SkTextBlobBuilderPriv is no longer needed
and is removed. For consistency, the allocRunRSXform overload is renamed
to allocRunTextRSXform.

Change-Id: I44be82d9038a433e1221d5cbfd8ed113ecb6d4fa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375017
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
7 files changed