The browser-process notifications are either missing or reusing images from the last notification. The reason is simple: In [atom_api_notification, we set the](base::TimeTicks::Now()) `icon_url` to `GURL()`, but in the notification presenter, [we assume an okay and unique path](https://github.com/electron/electron/blob/master/brightray/browser/win/notification_presenter_win.cc#L68-L73).

* Electron version: 1.7+
* Operating system: Windows

### Expected behavior
Notifications should contain the passed icon

### Actual behavior
Notifications contain either no or the last passed icon

### How to reproduce
Use browser-process notifications and attach an image on Windows 10 as icon. Now, send another notification with a different image. Observe how the first icon is shown.

brightray/browser/win/notification_presenter_win.cc
#include "brightray/browser/win/notification_presenter_win.h"
#include <string>
#include <vector>
#include "base/files/file_util.h"
#include "base/md5.h"
#include "base/strings/utf_string_conversions.h"
#include "base/win/windows_version.h"
#include "brightray/browser/win/notification_presenter_win7.h"
#include "brightray/browser/win/windows_toast_notification.h"
#include "content/public/browser/desktop_notification_delegate.h"
#include "content/public/common/platform_notification_data.h"
#include "third_party/skia/include/core/SkBitmap.h"
#include "ui/gfx/codec/png_codec.h"
#pragma comment(lib, "runtimeobject.lib")
namespace brightray {
namespace {
bool SaveIconToPath(const SkBitmap& bitmap, const base::FilePath& path) {