Add ptr_util.h from base until std::make_unique<> available

third_party/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/arm.gni")
 import("//build_overrides/build.gni")
 import("../pdfium.gni")
 
 group("third_party") {
   deps = [
@@ skipping 462 matching lines @@
     "//build/config/compiler:no_chromium_code",
     ":pdfium_third_party_config",
   ]
   sources = [
     "base/logging.h",
     "base/macros.h",
     "base/numerics/safe_conversions.h",
     "base/numerics/safe_conversions_impl.h",
     "base/numerics/safe_math.h",
     "base/numerics/safe_math_impl.h",
+    "base/ptr_util.h",
     "base/stl_util.h",
   ]
 }