[shaper] Correct flags for optional member functions

SkShaper does not publically depend on SkUnicode so should not re-export
the SkUnicode defines. Instead it should declare availability itself.

Change-Id: I7c9dc8397dc3c579f7a7f9c90e9f956c3fb1670c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/753617
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
5 files changed