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

mojo/public/cpp/bindings/lib/validation_context.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_VALIDATION_CONTEXT_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_VALIDATION_CONTEXT_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 13 matching lines @@
 class ValidationContext {
  public:
   // [data, data + data_num_bytes) specifies the initial valid memory range.
   // [0, num_handles) specifies the initial valid range of handle indices.
   //
   // If provided, |message| and |description| provide additional information
   // to use when reporting validation errors. In addition if |message| is
   // 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 = "");
 
   ~ValidationContext();
 
   // Claims the specified memory range.
   // The method succeeds if the range is valid to claim. (Please see
   // the comments for IsValidRange().)
   // On success, the valid memory range is shrinked to begin right after the end
@@ skipping 28 matching lines @@
   uint32_t handle_begin_;
   uint32_t handle_end_;
 
   DISALLOW_COPY_AND_ASSIGN(ValidationContext);
 };
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_VALIDATION_CONTEXT_H_