NewMessageListView: Fix scroll position.

This CL fixes the bug that the scroll position is wrong in such cases:
1. Initially there's no notification.
2. Open SystemTray
3. Press CapsLock
4. Bottom position of notification is wrong

TEST=NewUnifiedMessageCenterTest.AddAndRemoveNotification
BUG=769219

Change-Id: I362b2497da1b89c9decca9cca8e8e2d735d1293d
Reviewed-on: https://chromium-review.googlesource.com/c/1286011
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600667}

ash/system/message_center/new_unified_message_center_view.cc

@@ -208,6 +208,11 @@ void NewUnifiedMessageCenterView::UpdateVisibility() {
 }
 
 void NewUnifiedMessageCenterView::ScrollToPositionFromBottom() {
+  // Following logic doesn't work when the view is invisible, because it uses
+  // the height of |scroller_|.
+  if (!visible())
+    return;
+
   // If the last notification is taller than |scroller_|, we should align the
   // top of the notification with the top of |scroller_|.
   const int last_notification_offset =

ash/system/message_center/new_unified_message_center_view_unittest.cc

@@ -68,7 +68,8 @@ class NewUnifiedMessageCenterViewTest : public AshTestBase,
   void OnViewPreferredSizeChanged(views::View* view) override {
     if (view->GetPreferredSize() == view->size())
       return;
-    view->SetBoundsRect(gfx::Rect(view->GetPreferredSize()));
+    view->SetBoundsRect(view->visible() ? gfx::Rect(view->GetPreferredSize())
+                                        : gfx::Rect());
     view->Layout();
     ++size_changed_count_;
   }
@@ -152,6 +153,9 @@ TEST_F(NewUnifiedMessageCenterViewTest, AddAndRemoveNotification) {
 
   auto id0 = AddNotification();
   EXPECT_TRUE(message_center_view()->visible());
+  EXPECT_EQ(3 * kUnifiedNotificationCenterSpacing,
+            GetScrollerContents()->height() -
+                GetScroller()->GetVisibleRect().bottom());
 
   MessageCenter::Get()->RemoveNotification(id0, true /* by_user */);
   AnimateToEnd();