Network Tray: Protect from edge case crash

Bug: 806446
Change-Id: Ibb34f1ea9c43315f61cf21a8680123f1d3bc6f21
Reviewed-on: https://chromium-review.googlesource.com/914847
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536783}

ash/system/network/network_state_list_detailed_view.cc

@@ -96,7 +96,8 @@ class NetworkStateListDetailedView::InfoBubble
   void OnMouseExited(const ui::MouseEvent& event) override {
     // Like the user switching bubble/menu, hide the bubble when the mouse
     // exits.
-    detailed_view_->ResetInfoBubble();
+    if (detailed_view_)
+      detailed_view_->ResetInfoBubble();
   }
 
   // BubbleDialogDelegateView: