Fix non-noise cpplint errors in //crypto.

crypto/random_unittest.cc

 // Copyright (c) 2012 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.
 
 #include "crypto/random.h"
 
 #include <stddef.h>
 
+#include <string>
+
 #include "base/strings/string_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 // Basic functionality tests. Does NOT test the security of the random data.
 
 // Ensures we don't have all trivial data, i.e. that the data is indeed random.
 // Currently, that means the bytes cannot be all the same (e.g. all zeros).
 bool IsTrivial(const std::string& bytes) {
   for (size_t i = 0; i < bytes.size(); i++) {
     if (bytes[i] != bytes[0]) {
       return false;
     }
   }
   return true;
 }
 
 TEST(RandBytes, RandBytes) {
   std::string bytes(16, '\0');
   crypto::RandBytes(base::WriteInto(&bytes, bytes.size()), bytes.size());
   EXPECT_TRUE(!IsTrivial(bytes));
 }