Replace skstd::monostate with std::monostate.

Change-Id: Ia3a2b65ba00468d79e37b0dced613423ca5b81cb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502779
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>

include/private/SkTLogic.h

@@ -19,9 +19,6 @@
 
 namespace skstd {
 
-// C++17, <variant>
-struct monostate {};
-
 // C++17, <type_traits>
 template<typename...> struct conjunction : std::true_type { };
 template<typename T> struct conjunction<T> : T { };

src/core/SkEnumerate.h

@@ -11,10 +11,9 @@
 #include <cstddef>
 #include <iterator>
 #include <tuple>
+#include <variant>
 
-#include "include/private/SkTLogic.h"
-
-template <typename Iter, typename C = skstd::monostate>
+template <typename Iter, typename C = std::monostate>
 class SkEnumerate {
     using Captured = decltype(*std::declval<Iter>());
     template <typename> struct is_tuple : std::false_type {};