headers: forward declare SkDOM, and include SkClipOp

Change-Id: I38041cd2c71a89604f35832415287e2356592efd
Reviewed-on: https://skia-review.googlesource.com/8925
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
6 files changed