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

src/gpu/GrRenderTargetPriv.h

Index: src/gpu/GrRenderTargetPriv.h
diff --git a/src/gpu/GrRenderTargetPriv.h b/src/gpu/GrRenderTargetPriv.h
index 59262f4b11229df0891dd3461476e16e24cb76fe..f514a3c00d65b821c02b3d629101db0e7d22a6b2 100644
--- a/src/gpu/GrRenderTargetPriv.h
+++ b/src/gpu/GrRenderTargetPriv.h
@@ -24,7 +24,7 @@ public:
      * If this render target already has a stencil buffer, return it. Otherwise attempt to attach
      * one.
      */
-    GrStencilAttachment* attachStencilAttachment() const;
+    GrStencilAttachment* attachStencilAttachment();
 
     void didAttachStencilAttachment(GrStencilAttachment*);