Move the empty glyph checks into the common code

This is part of a larger movement to remove unneeded
cache API chatter.

Change-Id: Id8e9c72a5cebe31a7efe51a947a1d4c68384609a
Reviewed-on: https://skia-review.googlesource.com/c/172947
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
7 files changed