[graphite] Allow graphite to compile without ganesh and add shared gn files.

With this you should be able to set skia_enable_gpu=false in your gn
args and still be able to compile and run graphite.

Bug: skia:12466
Change-Id: Ibffc8774a8c46afad3f717dabc54fa831338a807
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/463317
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
5 files changed