Revert "SaveLayerRec::fClipMask -> raw pointer"

Revert "SaveLayerRec::fClipMask -> raw pointer"

This reverts commit a6b72cb5729306fdd676d739c1e2c53afa0786a7.

Reason for revert: red bots

Original change's description:
> SaveLayerRec::fClipMask -> raw pointer
>
> Use raw pointers for optional clip mask plumbing, to match the backdrop
> API.
>
> Change-Id: I7eb0ee5896faf34cc05789ba0703f35a4ab6a4f2
> Reviewed-on: https://skia-review.googlesource.com/14901
> Reviewed-by: Mike Reed <reed@google.com>
> Commit-Queue: Florin Malita <fmalita@chromium.org>
>

TBR=fmalita@chromium.org,reed@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I4387620cdc3410018af9cef221e5cf8d09015380
Reviewed-on: https://skia-review.googlesource.com/14955
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>