remove PLAIN_ENUM flag for SkClipOp

BUG=skia:

Change-Id: I4fc6f270582bb02218144098427bc356b715c893
Reviewed-on: https://skia-review.googlesource.com/6083
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed