Use random number as id in CallbacksRegistry.

It's very possible that the callbacks got GCed before the render view is
closed (like page getting refreshed), so we should not let browser call
the wrong callback, instead we should throw error whenever a callback is
not found.

common/api/lib/callbacks-registry.coffee

@@ -1,15 +1,19 @@
 module.exports =
 class CallbacksRegistry
   constructor: ->
-    @nextId = 0
+    @emptyFunc = -> throw new Error "Browser trying to call a non-exist callback
+      in renderer, this usually happens when renderer code forgot to release
+      a callback installed on objects in browser when renderer was going to be
+      unloaded or released."
     @callbacks = {}
 
   add: (callback) ->
-    @callbacks[++@nextId] = callback
-    @nextId
+    id = Math.random().toString()
+    @callbacks[id] = callback
+    id
 
   get: (id) ->
-    @callbacks[id]
+    @callbacks[id] ? ->
 
   call: (id, args...) ->
     @get(id).call global, args...