Convert JsonWriter::Write to taking a const ref for the in-param

pdf/pdfium/pdfium_engine.cc

Index: pdf/pdfium/pdfium_engine.cc
diff --git a/pdf/pdfium/pdfium_engine.cc b/pdf/pdfium/pdfium_engine.cc
index 8012dd265ec64b77513df3bbf0443f0e0d8e2835..d5e7a8f01cf8b4aed9afaf0b9729edaf6605d2ac 100644
--- a/pdf/pdfium/pdfium_engine.cc
+++ b/pdf/pdfium/pdfium_engine.cc
@@ -2440,7 +2440,7 @@ std::string PDFiumEngine::GetPageAsJSON(int index) {
   scoped_ptr<base::Value> node(
       pages_[index]->GetAccessibleContentAsValue(current_rotation_));
   std::string page_json;
-  base::JSONWriter::Write(node.get(), &page_json);
+  base::JSONWriter::Write(*node, &page_json);
   return page_json;
 }