SkPDF: add drop() virtual to release resources early.

bench/PDFBench.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Benchmark.h"
 #include "Resources.h"
 #include "SkData.h"
@@ skipping 11 matching lines @@
     size_t fN;
 };
 
 static void test_pdf_object_serialization(SkPDFObject* object) {
     // SkDebugWStream wStream;
     NullWStream wStream;
     SkPDFSubstituteMap substitutes;
     SkPDFObjNumMap objNumMap;
     objNumMap.addObjectRecursively(object, substitutes);
     for (int i = 0; i < objNumMap.objects().count(); ++i) {
-        SkPDFObject* object = objNumMap.objects()[i];
+        SkPDFObject* object = objNumMap.objects()[i].get();
         wStream.writeDecAsText(i + 1);
         wStream.writeText(" 0 obj\n");
         object->emitObject(&wStream, objNumMap, substitutes);
         wStream.writeText("\nendobj\n");
     }
 }
 
 class PDFImageBench : public Benchmark {
 public:
     PDFImageBench() {}
@@ skipping 119 matching lines @@
             (void)SkPDFUtils::FloatToDecimal(f, dst);
         }
     }
 };
 
 }  // namespace
 DEF_BENCH(return new PDFImageBench;)
 DEF_BENCH(return new PDFJpegImageBench;)
 DEF_BENCH(return new PDFCompressionBench;)
 DEF_BENCH(return new PDFScalarBench;)