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

bench/PDFBench.cpp

Index: bench/PDFBench.cpp
diff --git a/bench/PDFBench.cpp b/bench/PDFBench.cpp
index b7f48daac8a2dfa4b8ed115dd92b432504bda04b..1b4793a108cb62a64281546d096160ee84183f01 100644
--- a/bench/PDFBench.cpp
+++ b/bench/PDFBench.cpp
@@ -29,7 +29,7 @@ static void test_pdf_object_serialization(SkPDFObject* object) {
     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);