Make SkShaper an interface.

The interface here is not ideal, but there will need to be some build
clean-up before it can be changed.

Change-Id: Ic4d55634405f4c8d9c194e4e6f368287c9669dcd
Reviewed-on: https://skia-review.googlesource.com/c/193036
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
10 files changed