[Not for commit] Test performance impact if string validation checks whether it is utf8.

mojo/public/cpp/bindings/lib/string_serialization.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_STRING_SERIALIZATION_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_STRING_SERIALIZATION_H_
 
 #include <stddef.h>
 #include <string.h>
 
+#include "base/metrics/histogram_macros.h"
 #include "mojo/public/cpp/bindings/lib/array_internal.h"
 #include "mojo/public/cpp/bindings/lib/serialization_forward.h"
 #include "mojo/public/cpp/bindings/lib/serialization_util.h"
 #include "mojo/public/cpp/bindings/string_data_view.h"
 #include "mojo/public/cpp/bindings/string_traits.h"
 
 namespace mojo {
 namespace internal {
 
 template <typename MaybeConstUserType>
@@ skipping 32 matching lines @@
     *output = result;
 
     CustomContextHelper<Traits>::TearDown(input, custom_context);
   }
 
   static bool Deserialize(String_Data* input,
                           UserType* output,
                           SerializationContext* context) {
     if (!input)
       return CallSetToNullIfExists<Traits>(output);
+    UMA_HISTOGRAM_COUNTS_100000("Experiment.MojomStringSize", input->size());
     return Traits::Read(StringDataView(input, context), output);
   }
 };
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_STRING_SERIALIZATION_H_