* 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.cc
}
AtomPermissionManager::~AtomPermissionManager() {
}
void AtomPermissionManager::SetPermissionRequestHandler(
    const RequestHandler& handler) {
  if (handler.is_null() && !pending_requests_.IsEmpty()) {
    for (PendingRequestsMap::const_iterator iter(&pending_requests_);
         !iter.IsAtEnd(); iter.Advance()) {
      auto request = iter.GetCurrentValue();
      if (!WebContentsDestroyed(request->render_process_id()))
        request->RunCallback();
    }
    pending_requests_.Clear();
  }
  request_handler_ = handler;
}
int AtomPermissionManager::RequestPermission(
    content::PermissionType permission,
    content::RenderFrameHost* render_frame_host,
    const GURL& requesting_origin,
    bool user_gesture,
    const StatusCallback& response_callback) {
  return RequestPermissions(
      std::vector<content::PermissionType>(1, permission),
      render_frame_host,
      requesting_origin,
      user_gesture,
      base::Bind(&PermissionRequestResponseCallbackWrapper, response_callback));
}
int AtomPermissionManager::RequestPermissions(