Fuchsia: fix build

runtime/bin/crypto_fuchsia.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_FUCHSIA)
 
 #include "bin/crypto.h"
 
 #include <magenta/syscalls.h>
 
 namespace dart {
 namespace bin {
 
 bool Crypto::GetRandomBytes(intptr_t count, uint8_t* buffer) {
   intptr_t read = 0;
   while (read < count) {
     const intptr_t remaining = count - read;
     const intptr_t len =
         (MX_CPRNG_DRAW_MAX_LEN < remaining) ? MX_CPRNG_DRAW_MAX_LEN : remaining;
-    mx_size_t res = 0;
+    size_t res = 0;
     const mx_status_t status = mx_cprng_draw(buffer + read, len, &res);
     if (status != NO_ERROR) {
       return false;
     }
     read += res;
   }
   return true;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_FUCHSIA)