Stage SkFontMgr factories to return smart pointers.

Stage SkFontMgr factories to return smart pointers.

All the SkFontMgr factories currently return bare pointers and sometimes
even document the ownership rules. Since such factories can be
implemented by external ports, the ownership rules should be explicit in
order to prevent simple reference counting issues.

Change-Id: I25b598ce0954cd473a3fb1f8adc0cb86331583ca
Reviewed-on: https://skia-review.googlesource.com/6533
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>