Initialize NotificationDispatcher::next_notification_id_ to 0.

content/child/notifications/notification_dispatcher.cc

Index: content/child/notifications/notification_dispatcher.cc
diff --git a/content/child/notifications/notification_dispatcher.cc b/content/child/notifications/notification_dispatcher.cc
index 609f5665ae4500aa9d7aa6ba3212dae64bdc5f48..9401903333dda9ce8d2c45aea5e9bf05e2c81ca6 100644
--- a/content/child/notifications/notification_dispatcher.cc
+++ b/content/child/notifications/notification_dispatcher.cc
@@ -15,7 +15,8 @@ namespace content {
 NotificationDispatcher::NotificationDispatcher(
     ThreadSafeSender* thread_safe_sender)
     : main_thread_loop_proxy_(base::MessageLoopProxy::current()),
-      thread_safe_sender_(thread_safe_sender) {
+      thread_safe_sender_(thread_safe_sender),
+      next_notification_id_(0) {
 }
 
 NotificationDispatcher::~NotificationDispatcher() {}