SkPDF/None: fix compilation

src/pdf/SkDocument_PDF_None.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "SkDocument.h"
 sk_sp<SkDocument> SkDocument::MakePDF(SkWStream*,
                                       SkScalar,
                                       const SkDocument::PDFMetadata&,
-                                      SkPixelSerializer*,
+                                      sk_sp<SkPixelSerializer>,
                                       bool) {
     return nullptr;
 }
 sk_sp<SkDocument> SkDocument::MakePDF(const char path[], SkScalar) {
     return nullptr;
 }