Network Service: Fix nullptr NavigationController for LoginHandler

This CL adds a nullptr check for NavigationController in
LoginHandler::Observe().

Bug: 817968
Change-Id: I08d9de516f7d3e788e644c38173ecb969ef8cb58
Reviewed-on: https://chromium-review.googlesource.com/944255Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Commit-Queue: Jun Cai <juncai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540322}

chrome/browser/ui/login/login_handler.cc

@@ -238,8 +238,8 @@ void LoginHandler::Observe(int type,
   // Ignore login notification events from other profiles.
   NavigationController* controller =
       content::Source<NavigationController>(source).ptr();
-  if (controller->GetBrowserContext() !=
-      requesting_contents->GetBrowserContext()) {
+  if (!controller || controller->GetBrowserContext() !=
+                         requesting_contents->GetBrowserContext()) {
     return;
   }