Add a MailNotifier that notifies chrome-security-bugs--@google.com for LKGR builds.

scripts/slave/gatekeeper.json

 {
   "comment": ["This is a configuration file for gatekeeper_ng.py",
               "Look at that for documentation on this file's format."],
 
   "masters": {
     "https://build.chromium.org/p/chromium": [
       {
         "categories": [
           "chromium_tree_closer"
         ],
@@ skipping 191 matching lines @@
 
         "builders": {
           "Win x64": {
             "categories": [
               "chromium_archive_build",
               "chromium_compile",
               "chromium_windows"
             ]
           }
         }
+      },
+      {
+        "tree_notify": [
+          "chrome-security-sheriff@grotations.appspotmail.com"
+        ],
+
+        "subject_template": "LKGR build failure on %(builder_name)s",
+
+        "forgive_all": true,
+        "builders": {
+          "*": {}
+        }
       }
     ],
     "https://build.chromium.org/p/chromium.mac": [
       {
         "categories": [
           "chromium_tree_closer"
         ],
         "builders": {
           "iOS_Device": {
             "sheriff_classes": ["sheriff_ios_europe", "sheriff_ios_us"]
@@ skipping 632 matching lines @@
       "forgiving_optional": [
         "svnkill",
         "taskkill"
       ],
       "closing_optional": [
         "compile"
       ]
     }
   }
 }