* Electron version: 1.7.4
* Operating system: macOS Sierra

### Expected behavior

Load the PDF file correctly.

### Actual behavior

When I try to load something like http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1118&context=podimproveacad, the '&' symbol seems to interrupt the PDF Viewer to load the file.
![](http://i.imgur.com/RDSeY2u.png)

You can see that part of query string behind '&' get chopped, and I believe this is the reason why the PDF Viewer failed to load.

### How to reproduce

Clone the reproducible project at https://github.com/qazbnm456/electron-quick-start.
```
$ yarn # install packages
$ yarn run start # start the electron
```
atom/browser/atom_web_ui_controller_factory.cc
content::BrowserContext* browser_context,
    const GURL& url) const {
  return UseWebUIForURL(browser_context, url);
}
content::WebUIController*
AtomWebUIControllerFactory::CreateWebUIControllerForURL(content::WebUI* web_ui,
                                                        const GURL& url) const {
  if (url.host() == kPdfViewerUIHost) {
    base::StringPairs toplevel_params;
    base::SplitStringIntoKeyValuePairs(url.query(), '=', '&', &toplevel_params);
    std::string stream_id, src;
    for (const auto& param : toplevel_params) {
      if (param.first == kPdfPluginSrc) {
        src = param.second;
      }
    }
    if (url.has_ref()) {
      src = src + '#' + url.ref();
    }
    auto browser_context = web_ui->GetWebContents()->GetBrowserContext();
    return new PdfViewerUI(browser_context, web_ui, src);
  }
  return nullptr;
}
}