Stub out libevent's signal.c.

In libevent, socketpair() is used. However, it will be prohibited
by seccomp-bpf in nacl_helper_nonsfi. As its preparation, this CL
stubs out signal.c, which is the only module calling socketpair().

TEST=Ran bots.
BUG=358417
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_chromium_trusty32_rel,linux_arm
TBR=darin@chromium.org

Review URL: https://codereview.chromium.org/1093113002

Cr-Commit-Position: refs/heads/master@{#327213}

third_party/libevent/README.chromium

@@ -24,3 +24,5 @@ static library using GYP.
    nacl_nonsfi/random.c is also added to provide the random() function,
    which is missing in the newlib-based PNaCl toolchain.
 8) Apply https://github.com/libevent/libevent/commit/ea6b1df
+9) Stub out signal.c for nacl_helper_nonsfi. socketpair() will be prohibited
+   by sandbox in nacl_helper_nonsfi.

third_party/libevent/libevent_nacl_nonsfi.gyp

@@ -19,11 +19,11 @@
             'evutil.c',
             'log.c',
             'poll.c',
-            'signal.c',
             'strlcpy.c',
             'nacl_nonsfi/config.h',
             'nacl_nonsfi/event-config.h',
             'nacl_nonsfi/random.c',
+            'nacl_nonsfi/signal_stub.c',
           ],
           'defines': [
             'HAVE_CONFIG_H',

third_party/libevent/nacl_nonsfi/signal_stub.c

+/*
+ * 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.
+ */
+
+/*
+ * In nacl_helper_nonsfi, socketpair() is unavailable. In libevent, it is used
+ * to notify of a signal handler invocation, which is unused in
+ * nacl_helper_nonsfi. Unfortunately, there is no macro to disable the feature,
+ * so we stub out the signal module entirely.
+ */
+
+
+#include <signal.h>
+#include <stdlib.h>
+#include <sys/queue.h>
+
+/* config.h must be included before any other libevent header is included. */
+#include "config.h"
+
+#include "third_party/libevent/event-internal.h"
+#include "third_party/libevent/event.h"
+#include "third_party/libevent/evsignal.h"
+
+
+struct event_base *evsignal_base = 0;
+
+int evsignal_init(struct event_base *base) {
+  /* Do nothing, and return success. */
+  return 0;
+}
+
+void evsignal_process(struct event_base *base) {
+}
+
+int evsignal_add(struct event *event) {
+  /* Do nothing, and return an error. */
+  return -1;
+}
+
+int evsignal_del(struct event *event) {
+  /* Do nothing, and return an error. */
+  return -1;
+}
+
+void evsignal_dealloc(struct event_base *base) {
+}