NotificationPlatformBridge should be a lot simpler. For example, Display() should only have a Notification and a Metadata argument, all considerations of profile and notification type are not relevant to native notification systems and should be a higher level concern (i.e. NativeNotificationDisplayService). As it stands, each NPB implementation has to duplicate a lot of code converting notification IDs and tracking active notifications. This work should be done in NativeNotificationDisplayService.

NativeNotificationDisplayService can implement a simple interface that acts as its API for each NPB implementation, along the lines of

class NotificationPlatformBridgeDelegate {
 public:
  virtual void HandleNotificationClosed(const std::string& id,
                                        bool by_user) = 0;
  virtual void HandleNotificationClicked(const std::string& id) = 0;
  virtual void HandleNotificationButtonClicked(const std::string& id,
                                       int button_index) = 0;
};

Connecting the events to the NotificationDisplayService for the correct profile, tracking the NotificationCommon::Type, etc., will then not be necessary for each NPB.