Refactor sandboxed renderer init scripts

This change gives sandboxed renderer scripts a similar structure to what already
exists in the lib/{browser,renderer,common} directories.

It also allows sandboxed renderer initialization to share more code with
non-sandboxed renderers (ipcRenderer is now imported directly from
lib/renderer/api/ipc-renderer).

lib/sandboxed_renderer/api/ipc-renderer.js

@@ -0,0 +1,20 @@
+const ipcRenderer = require('../../renderer/api/ipc-renderer')
+
+const v8Util = process.atomBinding('v8_util')
+const ipcNative = process.atomBinding('ipc')
+
+// AtomSandboxedRendererClient will look for the "ipcNative" hidden object when
+// invoking the `onMessage`/`onExit` callbacks. We could reuse "ipc" and assign
+// `onMessage`/`onExit` directly to `ipcRenderer`, but it is better to separate
+// private/public APIs.
+v8Util.setHiddenValue(global, 'ipcNative', ipcNative)
+
+ipcNative.onMessage = function (channel, args) {
+  ipcRenderer.emit(channel, {sender: ipcRenderer}, ...args)
+}
+
+ipcNative.onExit = function () {
+  process.emit('exit')
+}
+
+module.exports = ipcRenderer