Only call browserify when one of sandbox sources change

lib/renderer/api/ipc-renderer-setup.js

@@ -1,3 +1,6 @@
+// Any requires added here need to be added to the browserify_entries array
+// in filenames.gypi so they get built into the preload_bundle.js bundle
+
 module.exports = function (ipcRenderer, binding) {
   ipcRenderer.send = function (...args) {
     return binding.send('ipc-message', args)

lib/sandboxed_renderer/init.js

@@ -1,3 +1,6 @@
+// Any requires added here need to be added to the browserify_entries array
+// in filenames.gypi so they get built into the preload_bundle.js bundle
+
 /* eslint no-eval: "off" */
 /* global binding, preloadPath, process, Buffer */
 const events = require('events')