* 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
#include "atom/browser/atom_web_ui_controller_factory.h"
#include <string>
#include "atom/browser/ui/webui/pdf_viewer_ui.h"
#include "atom/common/atom_constants.h"
#include "base/strings/string_split.h"
#include "base/strings/string_util.h"
#include "content/public/browser/web_contents.h"
namespace atom {
AtomWebUIControllerFactory* AtomWebUIControllerFactory::GetInstance() {
  return base::Singleton<AtomWebUIControllerFactory>::get();
}
AtomWebUIControllerFactory::AtomWebUIControllerFactory() {}
AtomWebUIControllerFactory::~AtomWebUIControllerFactory() {}
content::WebUI::TypeID AtomWebUIControllerFactory::GetWebUIType(
    content::BrowserContext* browser_context,
    const GURL& url) const {
  if (url.host() == kPdfViewerUIHost) {
    return const_cast<AtomWebUIControllerFactory*>(this);
  }
  return content::WebUI::kNoWebUI;
}
bool AtomWebUIControllerFactory::UseWebUIForURL(
    content::BrowserContext* browser_context,
    const GURL& url) const {
  return GetWebUIType(browser_context, url) != content::WebUI::kNoWebUI;
}
bool AtomWebUIControllerFactory::UseWebUIBindingsForURL(