Dynamically generate dependencies of browserify build actions

Instead of having to list in filenames.gypi every javascript file that may go
into a browserify build, generate this list dynamically when the build files are
created by gyp.

lib/sandboxed_renderer/init.js

@@ -1,6 +1,3 @@
-// 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, Buffer */
 const events = require('events')