Reset the default work_area when start observing.

In OOBE screen, shelf is not yet ready fully so that work_area_
isn't set well. It needs to be initialized here.

BUG=401743
R=stevenjb@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/544883002

Cr-Commit-Position: refs/heads/master@{#293435}

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();