Add an assertion to check that Chrome/Breakpad does not install a signal handler

src/native_client/src/trusted/service_runtime/nacl_signal.h

Index: src/native_client/src/trusted/service_runtime/nacl_signal.h
diff --git a/src/native_client/src/trusted/service_runtime/nacl_signal.h b/src/native_client/src/trusted/service_runtime/nacl_signal.h
index 2c947f6aa91a9459e5cdf180fbf4a55ae92485f2..002d1bdba2a0a462fc53a9a738fe8d65a1c8f2b7 100644
--- a/src/native_client/src/trusted/service_runtime/nacl_signal.h
+++ b/src/native_client/src/trusted/service_runtime/nacl_signal.h
@@ -100,6 +100,11 @@ void NaClSignalHandlerInit(void);
 void NaClSignalHandlerFini(void);
 
 /*
+ * Assert that no signal handlers are registered.
+ */
+void NaClSignalAssertNoHandlers(void);
+
+/*
  * Provides a signal safe method to write to stderr.
  */
 ssize_t NaClSignalErrorMessage(const char *str);