* Electron version: 1.8.2beta5
* Operating system: Windows 32-bit

### Expected behavior

After closing devtools, OSR rendered window should remain transparent. This is the case with 1.7.12 (latest 1.7 version as of this bug report)

### Actual behavior

OSR rendered window is filled with white background.

### How to reproduce

`electron main.js` (with the following two files stuck somewhere). dev tools will open themselves. OSR onpaint gets the correct transparent window, until devtools is closed, at which point, background becomes permanently white, destroying transparency.

main.js:
```
const {app, BrowserWindow} = require('electron')
const fs = require('fs');

app.disableHardwareAcceleration()

var counter = 0;

let win
app.once('ready', () => {
  win = new BrowserWindow({
    webPreferences: {
      offscreen: true,
    },
    show: false,
    transparent: true,
    width: 800,
    height: 600,
    frame: false,
  })
  win.loadURL(`file://${__dirname}/index.html`);
  win.webContents.on('paint', (event, dirty, image) => {
    // updateBitmap(dirty, image.getBitmap())
    ++counter;
    fs.writeFile('bitmap' + counter + '.raw', image.getBitmap(), (err) => {
      if (err) throw err;
      console.log('It\'s saved!');
    });
  })
  win.webContents.setFrameRate(30)
  win.toggleDevTools();
})
```

index.html:
```
<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>OSR test</title>
        <style>
          html, body { background: rgba(0, 0, 0, 0); }
       </style>
    </head>
    <body><font color=#ffffff>testing</font></body>
</html>
```
atom/renderer/renderer_client_base.cc
content::RenderFrame* render_frame,
    const std::string& mime_type,
    const GURL& original_url) {
  if (mime_type == content::kBrowserPluginMimeType) {
    return new GuestViewContainer(render_frame);
  } else {
    return nullptr;
  }
}
void RendererClientBase::AddSupportedKeySystems(
    std::vector<std::unique_ptr<::media::KeySystemProperties>>* key_systems) {
  AddChromeKeySystems(key_systems);
}
v8::Local<v8::Context> RendererClientBase::GetContext(
    blink::WebLocalFrame* frame, v8::Isolate* isolate) {
  if (isolated_world())
    return frame->WorldScriptContext(isolate, World::ISOLATED_WORLD);
  else
    return frame->MainWorldScriptContext();
}
}