Instead use std::unique_ptr to manage GrOp lifetime.
Change-Id: Ic1dc1e0ffd7254c3994221f498677af5bbf66a71
Reviewed-on: https://skia-review.googlesource.com/6479
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>