Make panels turn on or off shadow effect.

chrome/browser/ui/panels/stacked_panel_collection.cc

Index: chrome/browser/ui/panels/stacked_panel_collection.cc
diff --git a/chrome/browser/ui/panels/stacked_panel_collection.cc b/chrome/browser/ui/panels/stacked_panel_collection.cc
index f9ba70da724a265840d6601604bcff56145c7223..b5422566e18a8fcbacb5a950a984cf6de8f30738 100644
--- a/chrome/browser/ui/panels/stacked_panel_collection.cc
+++ b/chrome/browser/ui/panels/stacked_panel_collection.cc
@@ -522,6 +522,9 @@ void StackedPanelCollection::UpdatePanelOnCollectionChange(Panel* panel) {
       static_cast<Panel::AttentionMode>(Panel::USE_PANEL_ATTENTION |
                                         Panel::USE_SYSTEM_ATTENTION));
   panel->SetAlwaysOnTop(false);
+  // TODO(jianli): Change from true to false when the background window is used
+  // to provide the shadow.
+  panel->ShowShadow(true);
   panel->EnableResizeByMouse(true);
   panel->UpdateMinimizeRestoreButtonVisibility();
   UpdatePanelCornerStyle(panel);