Add MakeUnique from chromium and change StunMessage::AddAttribute to take a unique_ptr.

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index 83d9ebc3da04d300905a3cca72a5dd7e5e34a253..7514b07c8df48ea08e78404b6c35d40e94f6327d 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -138,6 +138,7 @@ rtc_static_library("rtc_base_approved") {
     "platform_thread.cc",
     "platform_thread.h",
     "platform_thread_types.h",
+    "ptr_util.h",
     "race_checker.cc",
     "race_checker.h",
     "random.cc",
@@ -849,6 +850,7 @@ if (rtc_include_tests) {
       "network_unittest.cc",
       "optionsfile_unittest.cc",
       "proxy_unittest.cc",
+      "ptr_util_unittest.cc",
       "ratelimiter_unittest.cc",
       "rollingaccumulator_unittest.cc",
       "rtccertificate_unittest.cc",