Implement update notifications for ChromeVox Next.

chrome/browser/resources/chromeos/chromevox/chromevox/background/keymaps/classic_keymap.json

 { "bindings": [
  {"command": "handleTabPrev",
   "sequence": {"keys": {"keyCode": [9],
                         "shiftKey": [true]}
               }},
 
  {"command": "handleTab",
   "sequence": {"keys": {"keyCode": [9]}
               }},
 
@@ skipping 342 matching lines @@
  {"command": "showKbExplorerPage",
   "sequence": {"cvoxModifier": true,
                "keys": {"keyCode": [79, 75]}
               }},
 
  {"command": "showOptionsPage",
   "sequence": {"cvoxModifier": true,
                "keys": {"keyCode": [79, 79]}
               }},
 
+ {"command": "showNextUpdatePage",
+  "sequence": {"cvoxModifier": true,
+               "keys": {"keyCode": [79, 78]}
+              }},
+
  {"command": "help",
   "sequence": {"cvoxModifier": true,
                "keys": {"keyCode": [79, 84]}
               }},
 
  {"command": "previousHeading1",
   "sequence": {"cvoxModifier": true,
                "keys": {"keyCode": [80, 49]}
               }},
 
@@ skipping 171 matching lines @@
   "sequence": {"cvoxModifier": true,
                "keys": {"keyCode": [68, 66]}
               }},
 
  {"command": "toggleSemantics",
   "sequence": {"cvoxModifier": true,
                "keys": {"keyCode": [77, 83]}
               }}
   ]
 }