* 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/atom_permission_manager.h
#ifndef ATOM_BROWSER_ATOM_PERMISSION_MANAGER_H_
#define ATOM_BROWSER_ATOM_PERMISSION_MANAGER_H_
#include <map>
#include <vector>
#include "base/callback.h"
#include "base/id_map.h"
#include "content/public/browser/permission_manager.h"
namespace content {
class WebContents;
}
namespace atom {
class AtomPermissionManager : public content::PermissionManager {