Implement rewrite logic for assigning a temporary scoped_refptr to T*.

tools/clang/rewrite_scoped_refptr/tests/test11-expected.cc

 // Copyright (c) 2013 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 <vector>
 
 #include "scoped_refptr.h"
 
 struct Foo {
   int dummy;
 };
 
 typedef std::vector<scoped_refptr<Foo> > FooList;
 
 void TestsAScopedRefptr() {
   FooList list;
   list.push_back(new Foo);
   list.push_back(new Foo);
   for (FooList::const_iterator it = list.begin(); it != list.end(); ++it) {
+    if (!it->get())
+      continue;
     Foo* item = it->get();
   }
 }