Add ENABLE_NATIVE_NOTIFICATIONS guard in browser_process_impl.h

The created_notification_bridge_ member variable is only used
when enable_native_notifications = true.

Bug: None
Change-Id: Iab1ff6864d9b600667c93178de24eda5eef656bd
Reviewed-on: https://chromium-review.googlesource.com/1015746Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552102}

chrome/browser/browser_process_impl.h

@@ -273,7 +273,10 @@ class BrowserProcessImpl : public BrowserProcess,
 
   std::unique_ptr<StatusTray> status_tray_;
 
+#if BUILDFLAG(ENABLE_NATIVE_NOTIFICATIONS)
   bool created_notification_bridge_ = false;
+#endif
+
   std::unique_ptr<NotificationPlatformBridge> notification_bridge_;
 
 #if BUILDFLAG(ENABLE_BACKGROUND_MODE)