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/exports/electron.js

@@ -0,0 +1,8 @@
+Object.defineProperties(exports, {
+  ipcRenderer: {
+    enumerable: true,
+    get: function () {
+      return require('../ipc-renderer')
+    }
+  }
+})