Migrate absl variant.h and utility.h in android_webview

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: I9ef5c483ad19da6d91c65090584419021338f9fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6343250
Reviewed-by: Richard (Torne) Coles <torne@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1430893}