* 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_resource_dispatcher_host_delegate.cc
bool allowed) {
  if (allowed)
    platform_util::OpenExternal(
#if defined(OS_WIN)
        base::UTF8ToUTF16(escaped_url.spec()),
#else
        escaped_url,
#endif
        true);
}
void HandleExternalProtocolInUI(
    const GURL& url,
    const content::ResourceRequestInfo::WebContentsGetter& web_contents_getter,
    bool has_user_gesture) {
  content::WebContents* web_contents = web_contents_getter.Run();
  if (!web_contents)
    return;
  auto permission_helper =
      WebContentsPermissionHelper::FromWebContents(web_contents);
  if (!permission_helper)
    return;
  GURL escaped_url(net::EscapeExternalHandlerValue(url.spec()));
  auto callback = base::Bind(&OnOpenExternal, escaped_url);
  permission_helper->RequestOpenExternalPermission(callback, has_user_gesture);
}
void OnPdfResourceIntercepted(