Fix clang -Wwritable-strings warnings in the Windows sandbox

sandbox/win/src/policy_engine_unittest.cc

 // Copyright (c) 2006-2008 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 "sandbox/win/src/policy_engine_params.h"
 #include "sandbox/win/src/policy_engine_processor.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 #define POLPARAMS_BEGIN(x) sandbox::ParameterSet x[] = {
 #define POLPARAM(p) sandbox::ParamPickerMake(p),
@@ skipping 42 matching lines @@
   opcode_maker.MakeOpAction(ASK_BROKER, kPolNone);
 
   // Add rule set #2
   opcode_maker.MakeOpWStringMatch(FileNameArg, L".TXT",
                                   kSeekToEnd, CASE_INSENSITIVE, kPolNone);
   opcode_maker.MakeOpNumberMatch(CreationDispositionArg, CREATE_NEW,
                                  kPolNegateEval);
   opcode_maker.MakeOpAction(FAKE_ACCESS_DENIED, kPolNone);
   policy->opcode_count = 7;
 
-  wchar_t* filename = L"c:\\Documents and Settings\\Microsoft\\BLAH.txt";
+  const wchar_t* filename = L"c:\\Documents and Settings\\Microsoft\\BLAH.txt";
   unsigned long creation_mode = OPEN_EXISTING;
   unsigned long flags = FILE_ATTRIBUTE_NORMAL;
   void* security_descriptor = NULL;
 
   POLPARAMS_BEGIN(eval_params)
     POLPARAM(filename)
     POLPARAM(creation_mode)
     POLPARAM(flags)
     POLPARAM(security_descriptor)
   POLPARAMS_END;
@@ skipping 19 matching lines @@
   // Changing creation_mode such that evaluation should match rule #2.
   creation_mode = OPEN_ALWAYS;
   pr = pol_ev.Evaluate(kShortEval, eval_params, _countof(eval_params));
   EXPECT_EQ(POLICY_MATCH, pr);
   EXPECT_EQ(FAKE_ACCESS_DENIED, pol_ev.GetAction());
 
   delete [] reinterpret_cast<char*>(policy);
 }
 
 }  // namespace sandbox