Add pdfium::MakeRetain<>() helper function.

core/fxcrt/cfx_retain_ptr_unittest.cpp

 // Copyright 2016 PDFium 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 "core/fxcrt/cfx_retain_ptr.h"
 
 #include <utility>
 
 #include "testing/fx_string_testhelpers.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 195 matching lines @@
 
 TEST(fxcrt, RetainPtrBool) {
   PseudoRetainable obj1;
   CFX_RetainPtr<PseudoRetainable> null_ptr;
   CFX_RetainPtr<PseudoRetainable> obj1_ptr(&obj1);
   bool null_bool = !!null_ptr;
   bool obj1_bool = !!obj1_ptr;
   EXPECT_FALSE(null_bool);
   EXPECT_TRUE(obj1_bool);
 }
+
+TEST(fxcrt, RetainPtrMakeRetained) {
+  auto ptr = pdfium::MakeRetain<CFX_Retainable>();
+  EXPECT_TRUE(ptr->HasOneRef());
+  {
+    CFX_RetainPtr<CFX_Retainable> other = ptr;
+    EXPECT_FALSE(ptr->HasOneRef());
+  }
+  EXPECT_TRUE(ptr->HasOneRef());
+}