Use NSS_DISABLE_UNLOAD in valgrind and heapcheck scripts to avoid unsymbolized backtraces.

tools/heapcheck/heapcheck_test.py

Index: tools/heapcheck/heapcheck_test.py
diff --git a/tools/heapcheck/heapcheck_test.py b/tools/heapcheck/heapcheck_test.py
index 2e86e81a8671042604ca2ed1e499f79b7946327c..7082f730995135d2ed2c8b16d3ede789715830d4 100644
--- a/tools/heapcheck/heapcheck_test.py
+++ b/tools/heapcheck/heapcheck_test.py
@@ -1,5 +1,5 @@
 #!/usr/bin/python
-# Copyright (c) 2006-2009 The Chromium Authors. All rights reserved.
+# Copyright (c) 2011 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.
 
@@ -44,6 +44,7 @@ class HeapcheckWrapper(object):
     proc += self._args
     self.PutEnvAndLog('G_SLICE', 'always-malloc')
     self.PutEnvAndLog('NSS_DISABLE_ARENA_FREE_LIST', '1')
+    self.PutEnvAndLog('NSS_DISABLE_UNLOAD', '1')
     self.PutEnvAndLog('GTEST_DEATH_TEST_USE_FORK', '1')
     self.PutEnvAndLog('HEAPCHECK', self._mode)
     self.PutEnvAndLog('HEAP_CHECK_MAX_LEAKS', '-1')