Use DCHECKs when observing only a single notification.

chrome/browser/web_applications/update_shortcut_worker_win.cc

Index: chrome/browser/web_applications/update_shortcut_worker_win.cc
diff --git a/chrome/browser/web_applications/update_shortcut_worker_win.cc b/chrome/browser/web_applications/update_shortcut_worker_win.cc
index cce42b26156dac76f7d876d634d621b0af2e012b..ddb0e9dcbd6234c0b1df85b3daa989818d740c7f 100644
--- a/chrome/browser/web_applications/update_shortcut_worker_win.cc
+++ b/chrome/browser/web_applications/update_shortcut_worker_win.cc
@@ -65,11 +65,12 @@ void UpdateShortcutWorker::Observe(
     int type,
     const content::NotificationSource& source,
     const content::NotificationDetails& details) {
-  if (type == chrome::NOTIFICATION_TAB_CLOSING &&
-      content::Source<NavigationController>(source).ptr() ==
-        &web_contents_->GetController()) {
+  DCHECK_EQ(chrome::NOTIFICATION_TAB_CLOSING, type);
+
+  if (content::Source<NavigationController>(source).ptr() ==
+      &web_contents_->GetController()) {
     // Underlying tab is closing.
-    web_contents_ = NULL;
+    web_contents_ = nullptr;
   }
 }