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

src/native_client/src/trusted/service_runtime/sel_main.c

Index: src/native_client/src/trusted/service_runtime/sel_main.c
diff --git a/src/native_client/src/trusted/service_runtime/sel_main.c b/src/native_client/src/trusted/service_runtime/sel_main.c
index b6234f23cacc4bd91424f525a582199d44ce946c..41473e620291ee8e4a0f50471e8db72d203d64fe 100644
--- a/src/native_client/src/trusted/service_runtime/sel_main.c
+++ b/src/native_client/src/trusted/service_runtime/sel_main.c
@@ -503,7 +503,11 @@ int main(int  argc,
   }
 
   /* Remove the signal handler if we are not using it. */
-  if (!handle_signals) NaClSignalHandlerFini();
+  if (!handle_signals) {
+    NaClSignalHandlerFini();
+    /* Sanity check. */
+    NaClSignalAssertNoHandlers();
+  }
 
   /*
    * Open both files first because (on Mac OS X at least)