Make mojo::Callback safe to copy across threads.

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

Index: mojo/public/cpp/bindings/tests/binding_callback_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/binding_callback_unittest.cc b/mojo/public/cpp/bindings/tests/binding_callback_unittest.cc
index 8dbdb8cc7556e61cf5648d4a31d9570f8b4de2a9..68aedde69fb6daa5b10fb1d42bc57bd748d687b3 100644
--- a/mojo/public/cpp/bindings/tests/binding_callback_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/binding_callback_unittest.cc
@@ -338,7 +338,7 @@ TEST_F(BindingCallbackTest, DeleteCallbackBeforeBindingDeathTest) {
 #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();