* Electron version: 1.8.2
* Operating system: MacOS, Windows10

### Expected behavior

ipc messages from the main process should be sent to the renderer process when both `sandbox` and `contextIsolation` are set.

### Actual behavior

ipc messages sent in listeners to webview events are not visible in the renderer process.

### How to reproduce

```
$ git clone https://gist.github.com/4f6c4450cff194838e746ce23e60116d.git
$ cd 4f6c4450cff194838e746ce23e60116d/
$ electron main.js 
```

The devtools will open and the console will log one message.  But the second message on the ipc `ping` channel is not observed.

Toggling either `sandbox` or `contextIsolation` to false allows the `ping` to travel to the renderer process.  In the 1.7.X branch setting both values to true still allows the message to pass through successfully.
atom/renderer/atom_sandboxed_renderer_client.cc
v8::Local<v8::String> key,
                                mate::Arguments* margs) {
  v8::Local<v8::Object> exports;
  std::string module_key = gin::V8ToString(isolate, key);
  mate::Dictionary cache(isolate, GetModuleCache(isolate));
  if (cache.Get(module_key.c_str(), &exports)) {
    return exports;
  }
  auto* mod = node::binding::get_builtin_module(module_key.c_str());
  if (!mod) {
    char errmsg[1024];
    snprintf(errmsg, sizeof(errmsg), "No such module: %s", module_key.c_str());
    margs->ThrowError(errmsg);
    return exports;
  }
  exports = v8::Object::New(isolate);
  DCHECK_EQ(mod->nm_register_func, nullptr);
  DCHECK_NE(mod->nm_context_register_func, nullptr);
  mod->nm_context_register_func(exports, v8::Null(isolate),
                                isolate->GetCurrentContext(), mod->nm_priv);
  cache.Set(module_key.c_str(), exports);
  return exports;
}
v8::Local<v8::Value> CreatePreloadScript(v8::Isolate* isolate,
                                         v8::Local<v8::String> preloadSrc) {
  return RendererClientBase::RunScript(isolate->GetCurrentContext(),
                                       preloadSrc);
}
class AtomSandboxedRenderFrameObserver : public AtomRenderFrameObserver {