Fold SkShadowFlags into SkShadowUtils

Client CL: http://ag/24679622

Change-Id: I36183b8ab61a154cda71cc1ffdb5c2cf7db0f929
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/753177
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Owners-Override: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
12 files changed