Regularize SkGlyphCache creation.

This allows no need for downcasting for specialized use
of SkScalerContext for the remote case. This allows
cache priming to be used in a single process.

BUG=skia:7515

Change-Id: I963a50e36af9deef5a3414fc8a4c94ccfc38deaf
Reviewed-on: https://skia-review.googlesource.com/115121
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
6 files changed