Add PanelOverflowStrip to handle panel overflow.

chrome/browser/ui/panels/panel_strip.h

Index: chrome/browser/ui/panels/panel_strip.h
diff --git a/chrome/browser/ui/panels/panel_strip.h b/chrome/browser/ui/panels/panel_strip.h
index ca35100a141c323e63d227e72cebe5435e00ed10..1692307508b2f429414aed5817a3e9694f3f979a 100644
--- a/chrome/browser/ui/panels/panel_strip.h
+++ b/chrome/browser/ui/panels/panel_strip.h
@@ -46,8 +46,8 @@ class PanelStrip : public PanelMouseWatcherObserver {
   void EndDragging(bool cancelled);
 
   // Invoked when a panel's expansion state changes.
-  void OnPanelExpansionStateChanged(Panel::ExpansionState old_state,
-                                    Panel::ExpansionState new_state);
+  void OnPanelExpansionStateChanged(Panel* panel,
+                                    Panel::ExpansionState old_state);
 
   // Invoked when the preferred window size of the given panel might need to
   // get changed.
@@ -70,6 +70,7 @@ class PanelStrip : public PanelMouseWatcherObserver {
   int num_panels() const { return panels_.size(); }
   bool is_dragging_panel() const;
   const Panels& panels() const { return panels_; }
+  gfx::Rect display_area() const { return display_area_; }
 
   int GetMaxPanelWidth() const;
   int GetMaxPanelHeight() const;