mojo: Add the size template parameter to ArrayTraits<base::span>

The ArrayTraits specialization for base::span with a specified size was
considered to be missing. This caused accessors in mojom traits that
returned a base::span with a specified size to fail to compile with:

error: static_assert failed due to requirement 'internal::AlwaysFalse<span<const unsigned char, 6> >::value' "Cannot find the mojo::ArrayTraits specialization. Did you forget to include the corresponding header file?"
Change-Id: I61ab07f2a1f74ea2eb5073b2776c995ce86b6255
Reviewed-on: https://chromium-review.googlesource.com/c/1356159
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612529}

mojo/public/cpp/bindings/array_traits_span.h

@@ -12,8 +12,8 @@
 
 namespace mojo {
 
-template <typename T>
-struct ArrayTraits<base::span<T>> {
+template <typename T, size_t Extent>
+struct ArrayTraits<base::span<T, Extent>> {
   using Element = T;
 
   // There is no concept of a null span, as it is indistinguishable from the

services/device/public/cpp/bluetooth/bluetooth_system_mojom_traits.h

@@ -17,7 +17,8 @@ template <>
 class StructTraits<device::mojom::BluetoothAddressDataView,
                    std::array<uint8_t, 6>> {
  public:
-  static base::span<const uint8_t> address(const std::array<uint8_t, 6>& addr) {
+  static base::span<const uint8_t, 6> address(
+      const std::array<uint8_t, 6>& addr) {
     return base::make_span(addr);
   }