Make mojo::Callback safe to copy across threads.

mojo/public/cpp/bindings/tests/binding_callback_unittest.cc

 // Copyright 2015 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 <stdint.h>
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
@@ skipping 320 matching lines @@
       ValueSaver(&last_client_callback_value_seen_, base::Closure()));
   run_loop.Run();
 
   // Check that server saw the correct value, but the client has not yet.
   EXPECT_EQ(7, server_impl.last_server_value_seen());
   EXPECT_EQ(0, last_client_callback_value_seen_);
 
 #if !defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)
   // Delete the callback without running it. This should cause a crash in debug
   // builds due to a DCHECK.
-  std::string regex("Check failed: !callback_was_dropped.");
+  std::string regex("Check failed: !is_valid");
 #if defined(OS_WIN)
   // TODO(msw): Fix MOJO_DCHECK logs and EXPECT_DEATH* on Win: crbug.com/535014
   regex.clear();
 #endif  // OS_WIN
   EXPECT_DEATH_IF_SUPPORTED(server_impl.DeleteCallback(), regex.c_str());
 #endif  // !defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)
 }
 
 }  // namespace
 }  // namespace test
 }  // namespace mojo