Dynamically create stencil buffer when needed.

src/gpu/GrSurfacePriv.h

Index: src/gpu/GrSurfacePriv.h
diff --git a/src/gpu/GrSurfacePriv.h b/src/gpu/GrSurfacePriv.h
index 3c1e9c1e957e670faa9ab5f795453368ec95155e..cd1f6b92babf9e2d32701166db5e621cabed66c3 100644
--- a/src/gpu/GrSurfacePriv.h
+++ b/src/gpu/GrSurfacePriv.h
@@ -34,9 +34,9 @@ public:
     bool hasPendingIO() const { return fSurface->hasPendingIO(); }
 
 private:
-    GrSurfacePriv(GrSurface* surface) : fSurface(surface) { }
-    GrSurfacePriv(const GrSurfacePriv& that) : fSurface(that.fSurface) { }
-    GrSurfacePriv& operator=(const GrSurface&); // unimpl
+    explicit GrSurfacePriv(GrSurface* surface) : fSurface(surface) {}
+    GrSurfacePriv(const GrSurfacePriv&); // unimpl
+    GrSurfacePriv& operator=(const GrSurfacePriv&); // unimpl
 
     // No taking addresses of this type.
     const GrSurfacePriv* operator&() const;