* 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_resource_dispatcher_host_delegate.cc
#include "atom/browser/atom_resource_dispatcher_host_delegate.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/login_handler.h"
#include "atom/browser/web_contents_permission_helper.h"
#include "atom/browser/web_contents_preferences.h"
#include "atom/common/atom_constants.h"
#include "atom/common/platform_util.h"
#include "base/strings/stringprintf.h"
#include "base/strings/utf_string_conversions.h"
#include "content/public/browser/browser_thread.h"
#include "content/public/browser/download_manager.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/browser/stream_info.h"
#include "net/base/escape.h"
#include "net/ssl/client_cert_store.h"
#include "net/url_request/url_request.h"
#include "url/gurl.h"
#if defined(USE_NSS_CERTS)
#include "net/ssl/client_cert_store_nss.h"
#elif defined(OS_WIN)
#include "net/ssl/client_cert_store_win.h"
#elif defined(OS_MACOSX)
#include "net/ssl/client_cert_store_mac.h"
#endif
using content::BrowserThread;
namespace atom {
namespace {
void OnOpenExternal(const GURL& escaped_url, bool allowed) {