Reset the default work_area when start observing.

ash/system/web_notification/ash_popup_alignment_delegate.cc

Index: ash/system/web_notification/ash_popup_alignment_delegate.cc
diff --git a/ash/system/web_notification/ash_popup_alignment_delegate.cc b/ash/system/web_notification/ash_popup_alignment_delegate.cc
index 1f8cd3a7bf2d1e59ca5debbe518614e883ff196a..a81b0e176ad3e73c842b6d2f2505b8c474b98ab5 100644
--- a/ash/system/web_notification/ash_popup_alignment_delegate.cc
+++ b/ash/system/web_notification/ash_popup_alignment_delegate.cc
@@ -51,6 +51,7 @@ void AshPopupAlignmentDelegate::StartObserving(gfx::Screen* screen,
                                                const gfx::Display& display) {
   screen_ = screen;
   display_id_ = display.id();
+  work_area_ = display.work_area();
   root_window_ = ash::Shell::GetInstance()->display_controller()->
       GetRootWindowForDisplayId(display_id_);
   UpdateShelf();