Unified: Change timing of stacking counter

This CL changes timing of stacking notification counter to appear in
UnifiedSystemTray. Combined with https://crrev.com/c/1215507, it fixes
top corner artifact of ARC++ notifications in UnifiedSystemTray.

TEST=manual
BUG=866777

Change-Id: I57f0ca846b129c18e7500d7738375644ff82157b
Reviewed-on: https://chromium-review.googlesource.com/1215602
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590177}

ash/message_center/message_list_view.cc

@@ -343,7 +343,7 @@ void MessageListView::SetBorderPadding() {
       gfx::Insets(message_center::kMarginBetweenItemsInList)));
 }
 
-int MessageListView::GetCountAboveVisibleRect() const {
+int MessageListView::GetCountAboveVisibleRect(int y_offset) const {
   DCHECK(scroller_);
 
   int height = 0;
@@ -358,7 +358,7 @@ int MessageListView::GetCountAboveVisibleRect() const {
     height += child->bounds().height() + padding;
     padding = GetMarginBetweenItems();
 
-    if (height >= scroller_->GetVisibleRect().y())
+    if (height >= scroller_->GetVisibleRect().y() + y_offset)
       return i;
   }
   return child_count();

ash/message_center/message_list_view.h

@@ -75,7 +75,7 @@ class ASH_EXPORT MessageListView
   void SetBorderPadding();
 
   // Get the number of notifications above ScrollView's visible rect.
-  int GetCountAboveVisibleRect() const;
+  int GetCountAboveVisibleRect(int y_offset) const;
 
   // Get the distance from the bottom of ScrollView's visible rect to the bottom
   // of the notification list.

ash/system/unified/unified_message_center_view.cc

@@ -193,7 +193,9 @@ void UnifiedMessageCenterView::LayoutInternal(bool force) {
     scroller_->SetBoundsRect(GetContentsBounds());
 
   bool was_visible = stacking_counter_->visible();
-  stacking_counter_->SetCount(message_list_view_->GetCountAboveVisibleRect());
+  int y_offset = was_visible ? 0 : kStackingNotificationCounterHeight;
+  stacking_counter_->SetCount(
+      message_list_view_->GetCountAboveVisibleRect(y_offset));
 
   // Only do the actual layout if counter visibility is changed or forced.
   if (was_visible != stacking_counter_->visible() || force) {