tracing: Make ConvertableToTraceFormat move-only scoped_ptr

cc/debug/traced_display_item_list.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 #include "cc/debug/traced_display_item_list.h"
 
 #include "base/json/json_writer.h"
 #include "cc/debug/traced_value.h"
 #include "cc/playback/display_item_list.h"
 
 namespace cc {
 
 TracedDisplayItemList::TracedDisplayItemList(
     scoped_refptr<const DisplayItemList> list,
     bool include_items)
     : display_item_list_(list), include_items_(include_items) {
 }
 
 TracedDisplayItemList::~TracedDisplayItemList() {
 }
 
 void TracedDisplayItemList::AppendAsTraceFormat(std::string* out) const {
-  scoped_refptr<base::trace_event::ConvertableToTraceFormat> convertable =
-      display_item_list_->AsValue(include_items_);
+  scoped_ptr<base::trace_event::ConvertableToTraceFormat> convertable(
+      display_item_list_->AsValue(include_items_));
   convertable->AppendAsTraceFormat(out);
 }
 
 }  // namespace cc