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

core/fxcrt/cfx_retain_ptr_unittest.cpp

Index: core/fxcrt/cfx_retain_ptr_unittest.cpp
diff --git a/core/fxcrt/cfx_retain_ptr_unittest.cpp b/core/fxcrt/cfx_retain_ptr_unittest.cpp
index aa92e57f32fb87e2d7702af4e06c8637385052f5..692beb82ffdf3eadd21d408007a2617ea31a5d85 100644
--- a/core/fxcrt/cfx_retain_ptr_unittest.cpp
+++ b/core/fxcrt/cfx_retain_ptr_unittest.cpp
@@ -213,3 +213,13 @@ TEST(fxcrt, RetainPtrBool) {
   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());
+}