Add a Retained Pointer smart class.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 5058b5b0a9fafc9f66545ff28309a75c76bf0c3e..5fa57eaf5e4c4909631959ed5faed5294d21df3c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -545,6 +545,7 @@ static_library("fxcrt") {
     "core/fxcrt/fxcrt_stream.cpp",
     "core/fxcrt/fxcrt_windows.cpp",
     "core/fxcrt/fxcrt_windows.h",
+    "core/fxcrt/include/cfx_retain_ptr.h",
     "core/fxcrt/plex.h",
     "core/fxcrt/xml_int.h",
     "core/include/fxcrt/fx_basic.h",
@@ -1476,6 +1477,7 @@ test("pdfium_unittests") {
     "core/fpdfdoc/doc_basic_unittest.cpp",
     "core/fpdftext/fpdf_text_int_unittest.cpp",
     "core/fxcodec/codec/fx_codec_jpx_unittest.cpp",
+    "core/fxcrt/cfx_retain_ptr_unittest.cpp",
     "core/fxcrt/fx_basic_bstring_unittest.cpp",
     "core/fxcrt/fx_basic_gcc_unittest.cpp",
     "core/fxcrt/fx_basic_memmgr_unittest.cpp",
@@ -1499,9 +1501,7 @@ test("pdfium_unittests") {
     ]
   }
   if (pdf_enable_v8) {
-    sources += [
-      "fpdfsdk/javascript/public_methods_unittest.cpp",
-    ]
+    sources += [ "fpdfsdk/javascript/public_methods_unittest.cpp" ]
     include_dirs += [
       "//v8",
       "//v8/include",