* Electron Version: 3.0.0-beta.1
* Operating System (Platform and Version): macOS 10.13.5
* Last known working Electron version: 2.0.3

**Expected Behavior**
Dev tools extension tabs should appear after being added with the `BrowserWindow.addDevToolsExtension` API. I specifically tested the React and Redux extensions. I can tell the React extension is getting installed but not being shown because the usual
> Download the React DevTools for a better development experience

warning is absent.

**Actual behavior**
Dev tools extension tabs do not appear.

**To Reproduce**
Manually obtain the extension paths following the instructions here:
https://electronjs.org/docs/tutorial/devtools-extension
and supply them to `addDevToolsExtensions`.

Alternately, use [electron-devtools-installer](https://github.com/MarshallOfSound/electron-devtools-installer), which I have a repro case for using just Redux here:
https://github.com/ajmacd/electron-quick-start-typescript

Working with 2.0.3:
```sh
$ git clone https://github.com/ajmacd/electron-quick-start-typescript -b 2.0.3-devtools-working 
$ cd electron-quick-start-typescript
$ npm install
$ npm start
```

Not working with 3.0.0-beta.1:
```sh
$ git checkout 3.0.0-beta.1-devtools-not-working
$ npm install
$ npm start
```
brightray/browser/devtools_embedder_message_dispatcher.cc
const base::DictionaryValue* dict;
  if (!value.GetAsDictionary(&dict))
    return false;
  int x = 0;
  int y = 0;
  int width = 0;
  int height = 0;
  if (!dict->GetInteger("x", &x) || !dict->GetInteger("y", &y) ||
      !dict->GetInteger("width", &width) ||
      !dict->GetInteger("height", &height))
    return false;
  rect->SetRect(x, y, width, height);
  return true;
}
template <typename T>
struct StorageTraits {
  using StorageType = T;
};
template <typename T>
struct StorageTraits<const T&> {
  using StorageType = T;
};
template <typename... Ts>
struct ParamTuple {
  bool Parse(const base::ListValue& list,
             const base::ListValue::const_iterator& it) {
    return it == list.end();
  }
  template <typename H, typename... As>
  void Apply(const H& handler, As... args) {