[fuchsia] Migrate absl variant.h and utility.h in fuchsia_web

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: I9203d2fb8316de81c37e99ba1582f28e6d14c28b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6343092
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1434199}

fuchsia_web/webengine/web_engine_main_delegate.cc

@@ -5,6 +5,7 @@
 #include "fuchsia_web/webengine/web_engine_main_delegate.h"
 
 #include <utility>
+#include <variant>
 
 #include "base/base_paths.h"
 #include "base/command_line.h"
@@ -115,7 +116,7 @@ std::optional<int> WebEngineMainDelegate::PreBrowserMain() {
   return std::nullopt;
 }
 
-absl::variant<int, content::MainFunctionParams>
+std::variant<int, content::MainFunctionParams>
 WebEngineMainDelegate::RunProcess(
     const std::string& process_type,
     content::MainFunctionParams main_function_params) {

fuchsia_web/webengine/web_engine_main_delegate.h

@@ -11,6 +11,7 @@
 #include <memory>
 #include <optional>
 #include <string>
+#include <variant>
 
 #include "content/public/app/content_main_delegate.h"
 #include "fuchsia_web/webengine/web_engine_export.h"
@@ -40,7 +41,7 @@ class WEB_ENGINE_EXPORT WebEngineMainDelegate
   std::optional<int> BasicStartupComplete() override;
   void PreSandboxStartup() override;
   std::optional<int> PreBrowserMain() override;
-  absl::variant<int, content::MainFunctionParams> RunProcess(
+  std::variant<int, content::MainFunctionParams> RunProcess(
       const std::string& process_type,
       content::MainFunctionParams main_function_params) override;
   content::ContentClient* CreateContentClient() override;