Turn NotificationPresenter into an abstract base class

This will allow us to have Mac-specific member variables without a bunch of
ifdefs.

brightray/browser/notification_presenter_mac.h

@@ -0,0 +1,23 @@
+#ifndef BRIGHTRAY_BROWSER_NOTIFICATION_PRESENTER_MAC_H_
+#define BRIGHTRAY_BROWSER_NOTIFICATION_PRESENTER_MAC_H_
+
+#import "browser/notification_presenter.h"
+
+#import "base/compiler_specific.h"
+
+namespace brightray {
+
+class NotificationPresenterMac : public NotificationPresenter {
+ public:
+  NotificationPresenterMac();
+  ~NotificationPresenterMac();
+
+  virtual void ShowNotification(
+      const content::ShowDesktopNotificationHostMsgParams&,
+      int render_process_id,
+      int render_view_id) OVERRIDE;
+};
+
+}
+
+#endif