* Electron version: 1.7.9
* Operating system: macOS 10.12.6

### Expected behavior

Often Electron apps want to get in front of `openExternal` calls to prevent them, or display a "are you sure you want to open" dialog for possibly unsafe url schemes like `ssh://`. At the moment, you can either blanket deny these or allow all for a give host via a `permissionRequestHandler`. It would be helpful for the permission handler to be given the url that the window is attempting to navigate to better determine the action it should take.


```
 $ git clone git@github.com:gerges/electron-quick-start.git -b  feat/set-permission-handler-openexternal-url
 $ npm install
 $ npm run start
```
atom/browser/web_contents_permission_helper.h
#ifndef ATOM_BROWSER_WEB_CONTENTS_PERMISSION_HELPER_H_
#define ATOM_BROWSER_WEB_CONTENTS_PERMISSION_HELPER_H_
#include "content/public/browser/permission_type.h"
#include "content/public/browser/web_contents_user_data.h"
#include "content/public/common/media_stream_request.h"
namespace atom {
class WebContentsPermissionHelper
    : public content::WebContentsUserData<WebContentsPermissionHelper> {
 public:
  ~WebContentsPermissionHelper() override;
  enum class PermissionType {