* 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
#include "ipc/ipc_channel.h"
#include "net/http/http_response_headers.h"
#include "net/url_request/url_fetcher.h"
#include "net/url_request/url_fetcher_response_writer.h"
#include "ui/display/display.h"
#include "ui/display/screen.h"
namespace brightray {
namespace {
const double kPresetZoomFactors[] = {0.25, 0.333, 0.5,  0.666, 0.75, 0.9,
                                     1.0,  1.1,   1.25, 1.5,   1.75, 2.0,
                                     2.5,  3.0,   4.0,  5.0};
const char kChromeUIDevToolsURL[] =
    "chrome-devtools://devtools/bundled/inspector.html?"
    "remoteBase=%s&"
    "can_dock=%s&"
    "toolbarColor=rgba(223,223,223,1)&"
    "textColor=rgba(0,0,0,1)&"
    "experiments=true";
const char kChromeUIDevToolsRemoteFrontendBase[] =
    "https://chrome-devtools-frontend.appspot.com/";
const char kChromeUIDevToolsRemoteFrontendPath[] = "serve_file";
const char kDevToolsBoundsPref[] = "brightray.devtools.bounds";
const char kDevToolsZoomPref[] = "brightray.devtools.zoom";
const char kDevToolsPreferences[] = "brightray.devtools.preferences";
const char kFrontendHostId[] = "id";
const char kFrontendHostMethod[] = "method";
const char kFrontendHostParams[] = "params";
const char kTitleFormat[] = "Developer Tools - %s";
const size_t kMaxMessageChunkSize = IPC::Channel::kMaximumMessageSize / 4;
void RectToDictionary(const gfx::Rect& bounds, base::DictionaryValue* dict) {