* 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
content::NavigationHandle* navigation_handle) {
  if (navigation_handle->IsInMainFrame() ||
      !navigation_handle->GetURL().SchemeIs("chrome-extension") ||
      !navigation_handle->HasCommitted())
    return;
  content::RenderFrameHost* frame = navigation_handle->GetRenderFrameHost();
  auto origin = navigation_handle->GetURL().GetOrigin().spec();
  auto it = extensions_api_.find(origin);
  if (it == extensions_api_.end())
    return;
  
  
  base::ReplaceFirstSubstringAfterOffset(&it->second, 0, "var chrome",
                                         "var chrome = window.chrome ");
  auto script = base::StringPrintf("%s(\"%s\")", it->second.c_str(),
                                   base::GenerateGUID().c_str());
  
  
  frame->ExecuteJavaScriptForTests(base::UTF8ToUTF16(script));
}
void InspectableWebContentsImpl::OnURLFetchComplete(