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).

electron.gyp

@@ -452,6 +452,8 @@
             'browserify',
             '--',
             'lib/sandboxed_renderer/init.js',
+            '-r',
+            './lib/sandboxed_renderer/api/exports/electron.js:electron',
             '-o',
             '<@(_outputs)',
           ],