Migrate absl variant.h and utility.h in dbus

Since https://crrev.com/c/6330348, some utils in
third_party/abseil-cpp/absl/types/variant.h and
and third_party/abseil-cpp/absl/utility/utility.h are only aliases for
their std counterparts. This CL migrates code to use std:: directly.

Bug: 40242126
Change-Id: Id2611ac7a68f34017f0ecad1097b6f2d9de5399d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6343191
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1431035}

dbus/values_util.cc

@@ -6,6 +6,7 @@
 
 #include <memory>
 #include <utility>
+#include <variant>
 
 #include "base/json/json_writer.h"
 #include "base/logging.h"
@@ -69,7 +70,7 @@ bool PopDictionaryEntries(MessageReader* reader,
 // Gets the D-Bus type signature for the value.
 std::string GetTypeSignature(base::ValueView value) {
   struct Visitor {
-    std::string operator()(absl::monostate) {
+    std::string operator()(std::monostate) {
       DLOG(ERROR) << "Unexpected type " << base::Value::Type::NONE;
       return std::string();
     }
@@ -220,7 +221,7 @@ void AppendBasicTypeValueData(MessageWriter* writer, base::ValueView value) {
   struct Visitor {
     raw_ptr<MessageWriter> writer;
 
-    void operator()(absl::monostate) {
+    void operator()(std::monostate) {
       DLOG(ERROR) << "Unexpected type: " << base::Value::Type::NONE;
     }
 
@@ -262,7 +263,7 @@ void AppendValueData(MessageWriter* writer, base::ValueView value) {
   struct Visitor {
     raw_ptr<MessageWriter> writer;
 
-    void operator()(absl::monostate) {
+    void operator()(std::monostate) {
       DLOG(ERROR) << "Unexpected type: " << base::Value::Type::NONE;
     }