* 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
#include "brightray/browser/devtools_embedder_message_dispatcher.h"
#include "base/bind.h"
#include "base/values.h"
namespace brightray {
namespace {
using DispatchCallback = DevToolsEmbedderMessageDispatcher::DispatchCallback;
bool GetValue(const base::Value& value, std::string* result) {
  return value.GetAsString(result);
}
bool GetValue(const base::Value& value, int* result) {
  return value.GetAsInteger(result);
}
bool GetValue(const base::Value& value, bool* result) {
  return value.GetAsBoolean(result);
}
bool GetValue(const base::Value& value, gfx::Rect* rect) {