* 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
double new_level = GetNextZoomLevel(GetDevToolsZoomLevel(), false);
  SetZoomLevelForWebContents(GetDevToolsWebContents(), new_level);
  UpdateDevToolsZoomLevel(new_level);
}
void InspectableWebContentsImpl::ZoomOut() {
  double new_level = GetNextZoomLevel(GetDevToolsZoomLevel(), true);
  SetZoomLevelForWebContents(GetDevToolsWebContents(), new_level);
  UpdateDevToolsZoomLevel(new_level);
}
void InspectableWebContentsImpl::ResetZoom() {
  SetZoomLevelForWebContents(GetDevToolsWebContents(), 0.);
  UpdateDevToolsZoomLevel(0.);
}
void InspectableWebContentsImpl::SetDevicesUpdatesEnabled(bool enabled) {}
void InspectableWebContentsImpl::DispatchProtocolMessageFromDevToolsFrontend(
    const std::string& message) {
  
  
  if (base::MatchPattern(message,
                         "{\"id\":*,"
                         "\"method\":\"Page.reload\","
                         "\"params\":*}")) {
    if (delegate_)
      delegate_->DevToolsReloadPage();
    return;
  }
  if (agent_host_.get())
    agent_host_->DispatchProtocolMessage(this, message);
}
void InspectableWebContentsImpl::SendJsonRequest(