* 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
bool docked) {
  if (managed_devtools_web_contents_)
    view_->SetIsDocked(docked);
  if (!callback.is_null())
    callback.Run(nullptr);
}
void InspectableWebContentsImpl::OpenInNewTab(const std::string& url) {}
void InspectableWebContentsImpl::SaveToFile(const std::string& url,
                                            const std::string& content,
                                            bool save_as) {
  if (delegate_)
    delegate_->DevToolsSaveToFile(url, content, save_as);
}
void InspectableWebContentsImpl::AppendToFile(const std::string& url,
                                              const std::string& content) {
  if (delegate_)
    delegate_->DevToolsAppendToFile(url, content);
}
void InspectableWebContentsImpl::RequestFileSystems() {
  if (delegate_)
    delegate_->DevToolsRequestFileSystems();
}
void InspectableWebContentsImpl::AddFileSystem(
    const std::string& file_system_path) {
  if (delegate_)
    delegate_->DevToolsAddFileSystem(
        base::FilePath::FromUTF8Unsafe(file_system_path));
}
void InspectableWebContentsImpl::RemoveFileSystem(