* 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 level) {
  content::HostZoomMap::SetZoomLevel(web_contents, level);
}
double GetNextZoomLevel(double level, bool out) {
  double factor = content::ZoomLevelToZoomFactor(level);
  size_t size = arraysize(kPresetZoomFactors);
  for (size_t i = 0; i < size; ++i) {
    if (!content::ZoomValuesEqual(kPresetZoomFactors[i], factor))
      continue;
    if (out && i > 0)
      return content::ZoomFactorToZoomLevel(kPresetZoomFactors[i - 1]);
    if (!out && i != size - 1)
      return content::ZoomFactorToZoomLevel(kPresetZoomFactors[i + 1]);
  }
  return level;
}
GURL GetRemoteBaseURL() {
  return GURL(base::StringPrintf("%s%s/%s/",
                                 kChromeUIDevToolsRemoteFrontendBase,
                                 kChromeUIDevToolsRemoteFrontendPath,
                                 content::GetWebKitRevision().c_str()));
}
GURL GetDevToolsURL(bool can_dock) {
  auto url_string = base::StringPrintf(kChromeUIDevToolsURL,
                                       GetRemoteBaseURL().spec().c_str(),
                                       can_dock ? "true" : "");
  return GURL(url_string);
}
class ResponseWriter : public net::URLFetcherResponseWriter {