[FIX] mail: error notification safari iphone 7

On safari iphone 7 (and possibly other browsers) we could have an error
by referencing to an undeclared Notification variable.

opw-726084

addons/mail/static/src/js/client_action.js

@@ -121,7 +121,7 @@ var ChatAction = Widget.extend(ControlPanelMixin, {
         "click .o_mail_request_permission": function (event) {
             event.preventDefault();
             this.$(".o_mail_annoying_notification_bar").slideUp();
-            var def = window.Notification.requestPermission();
+            var def = window.Notification && window.Notification.requestPermission();
             if (def) {
                 def.then(function (value) {
                     if (value === 'denied') {

addons/mail/static/src/js/utils.js

@@ -7,7 +7,7 @@ var web_client = require('web.web_client');
 
 
 function send_notification(title, content) {
-    if (Notification && Notification.permission === "granted") {
+    if (window.Notification && Notification.permission === "granted") {
         if (bus.is_master) {
             _send_native_notification(title, content);
         }