Move some of the adding stencil attachment logic of Gpu and into Render Target.

src/gpu/GrRenderTargetPriv.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrRenderTargetPriv_DEFINED
 #define GrRenderTargetPriv_DEFINED
 
 #include "GrRenderTarget.h"
 
 /** Class that adds methods to GrRenderTarget that are only intended for use internal to Skia.
     This class is purely a privileged window into GrRenderTarget. It should never have additional
     data members or virtual methods. */
 class GrRenderTargetPriv {
 public:
     /**
      * GrStencilAttachment is not part of the public API.
      */
     GrStencilAttachment* getStencilAttachment() const { return fRenderTarget->fStencilAttachment; }
 
     /**
      * If this render target already has a stencil buffer, return it. Otherwise attempt to attach
      * one.
      */
-    GrStencilAttachment* attachStencilAttachment() const;
+    GrStencilAttachment* attachStencilAttachment();
 
     void didAttachStencilAttachment(GrStencilAttachment*);
 
 private:
     explicit GrRenderTargetPriv(GrRenderTarget* renderTarget) : fRenderTarget(renderTarget) {}
     GrRenderTargetPriv(const GrRenderTargetPriv&) {} // unimpl
     GrRenderTargetPriv& operator=(const GrRenderTargetPriv&); // unimpl
 
     // No taking addresses of this type.
     const GrRenderTargetPriv* operator&() const;
     GrRenderTargetPriv* operator&();
 
     GrRenderTarget* fRenderTarget;
         
     friend class GrRenderTarget; // to construct/copy this type.
 };
 
 inline GrRenderTargetPriv GrRenderTarget::renderTargetPriv() { return GrRenderTargetPriv(this); }
 
 inline const GrRenderTargetPriv GrRenderTarget::renderTargetPriv () const {
     return GrRenderTargetPriv(const_cast<GrRenderTarget*>(this));
 }
 
 #endif