mojo: Fix warning (treated as error) on Windows perf bots

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

Index: mojo/public/cpp/bindings/lib/validation_context.h
diff --git a/mojo/public/cpp/bindings/lib/validation_context.h b/mojo/public/cpp/bindings/lib/validation_context.h
index 5b02a59fdfc04c4dd308cafbd6162b3a36a036fd..6045ca82acc0205df18c1f8432540ce00e924f20 100644
--- a/mojo/public/cpp/bindings/lib/validation_context.h
+++ b/mojo/public/cpp/bindings/lib/validation_context.h
@@ -31,7 +31,7 @@ class ValidationContext {
   // provided, the MojoNotifyBadMessage API will be used to notify the system of
   // such errors.
   ValidationContext(const void* data,
-                    uint32_t data_num_bytes,
+                    size_t data_num_bytes,
                     size_t num_handles,
                     Message* message = nullptr,
                     const base::StringPiece& description = "");