Make SkFontMgr_New_Custom_Data public

Surface SkFontMgr_New_Custom_Data in include/ports/SkFontMgr_data.h,
similar to SkFontMgr_directory.h etc.

While at it, update the API to use SkSpan instead of rawptr + count.

Change-Id: Id340762d0787d726f917b14af9bbf688de5d1486
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/656217
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
5 files changed