libstdc++: fix namespace of nullptr_t in array_buffer_view_helpers.h

nullptr_t is defined in namespace std.

Bug: 819294
Change-Id: I16fd876b84aff4f3c0ef6d4654b8ad5015e52d27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2127369Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754562}

third_party/blink/renderer/core/typed_arrays/array_buffer_view_helpers.h

@@ -36,7 +36,7 @@ class NotShared {
   template <typename U, std::enable_if_t<std::is_base_of<T, U>::value, int> = 0>
   NotShared(const NotShared<U>& other) : typed_array_(other.Get()) {}
 
-  explicit NotShared(nullptr_t) {}
+  explicit NotShared(std::nullptr_t) {}
   explicit NotShared(T* typed_array) : typed_array_(typed_array) {
     DCHECK(!typed_array || !typed_array->IsShared());
   }
@@ -95,7 +95,7 @@ class MaybeShared {
   template <typename U, std::enable_if_t<std::is_base_of<T, U>::value, int> = 0>
   MaybeShared(const MaybeShared<U>& other) : typed_array_(other.Get()) {}
 
-  explicit MaybeShared(nullptr_t) {}
+  explicit MaybeShared(std::nullptr_t) {}
   // [AllowShared] array buffer view may be a view of non-shared array buffer,
   // so we don't check if the buffer is SharedArrayBuffer or not.
   // https://heycam.github.io/webidl/#AllowShared