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

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

Index: mojo/public/cpp/bindings/lib/string_serialization.h
diff --git a/mojo/public/cpp/bindings/lib/string_serialization.h b/mojo/public/cpp/bindings/lib/string_serialization.h
index 6e0c75857690af53b1a11dd7cdbe70c0b6fea6c5..83be04a4b84e27337796df476b0a9a9048b25522 100644
--- a/mojo/public/cpp/bindings/lib/string_serialization.h
+++ b/mojo/public/cpp/bindings/lib/string_serialization.h
@@ -8,6 +8,7 @@
 #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"
@@ -60,6 +61,7 @@ struct Serializer<StringDataView, MaybeConstUserType> {
                           SerializationContext* context) {
     if (!input)
       return CallSetToNullIfExists<Traits>(output);
+    UMA_HISTOGRAM_COUNTS_100000("Experiment.MojomStringSize", input->size());
     return Traits::Read(StringDataView(input, context), output);
   }
 };