switched GrClipEdge to an enum class

Bug: skia:
Change-Id: Idf41580314a32739c70721530fc3ca48e566b044
Reviewed-on: https://skia-review.googlesource.com/70023
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
18 files changed