* 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/inspectable_web_contents_impl.cc
if (agent_host_.get())
    agent_host_->DetachClient(this);
  agent_host_ = nullptr;
}
void InspectableWebContentsImpl::CallClientFunction(
    const std::string& function_name,
    const base::Value* arg1,
    const base::Value* arg2,
    const base::Value* arg3) {
  if (!GetDevToolsWebContents())
    return;
  std::string javascript = function_name + "(";
  if (arg1) {
    std::string json;
    base::JSONWriter::Write(*arg1, &json);
    javascript.append(json);
    if (arg2) {
      base::JSONWriter::Write(*arg2, &json);
      javascript.append(", ").append(json);
      if (arg3) {
        base::JSONWriter::Write(*arg3, &json);
        javascript.append(", ").append(json);
      }
    }
  }
  javascript.append(");");
  GetDevToolsWebContents()->GetMainFrame()->ExecuteJavaScript(
      base::UTF8ToUTF16(javascript));
}
gfx::Rect InspectableWebContentsImpl::GetDevToolsBounds() const {