Embed setup bundle for preload scripts in sandboxed renderers.

Add a gyp target that creates a browserify bundle starting with
`lib/sandboxed_renderer/init.js`, which is embedded into the executable using
the `atom_js2c` target.

The goal of this bundle is to provide a very basic environment for preload
scripts where a `require` function is available.

lib/sandboxed_renderer/init.js

@@ -0,0 +1,56 @@
+/* eslint no-eval: "off" */
+/* global binding, preloadPath, process, Buffer */
+const events = require('events')
+
+const ipcRenderer = new events.EventEmitter()
+const proc = new events.EventEmitter()
+// eval in window scope:
+// http://www.ecma-international.org/ecma-262/5.1/#sec-10.4.2
+const geval = eval
+
+require('../renderer/api/ipc-renderer-setup')(ipcRenderer, binding)
+
+binding.onMessage = function (channel, args) {
+  ipcRenderer.emit.apply(ipcRenderer, [channel].concat(args))
+}
+
+binding.onExit = function () {
+  proc.emit('exit')
+}
+
+const preloadModules = new Map([
+  ['electron', {
+    ipcRenderer: ipcRenderer
+  }]
+])
+
+function preloadRequire (module) {
+  if (preloadModules.has(module)) {
+    return preloadModules.get(module)
+  }
+  throw new Error('module not found')
+}
+
+// Fetch the source for the preload
+let preloadSrc = ipcRenderer.sendSync('ELECTRON_BROWSER_READ_FILE', preloadPath)
+if (preloadSrc.err) {
+  throw new Error(preloadSrc.err)
+}
+
+// Wrap the source into a function receives a `require` function as argument.
+// Browserify bundles can make use of this, as explained in:
+// https://github.com/substack/node-browserify#multiple-bundles
+//
+// For example, the user can create a browserify bundle with:
+//
+//     $ browserify -x electron preload.js > renderer.js
+//
+// and any `require('electron')` calls in `preload.js` will work as expected
+// since browserify won't try to include `electron` in the bundle and will fall
+// back to the `preloadRequire` function above.
+let preloadWrapperSrc = `(function(require, process, Buffer, global) {
+${preloadSrc.data}
+})`
+
+let preloadFn = geval(preloadWrapperSrc)
+preloadFn(preloadRequire, proc, Buffer, global)