[ash-md] Implements a shadow over overview items and selector

ash/common/wm/overview/window_grid.h

Index: ash/common/wm/overview/window_grid.h
diff --git a/ash/common/wm/overview/window_grid.h b/ash/common/wm/overview/window_grid.h
index 04b8b9e150c393b8e445aca3354bd5625f32b183..f9678ff483ac6bc9d19e890830be2d19ab10fd19 100644
--- a/ash/common/wm/overview/window_grid.h
+++ b/ash/common/wm/overview/window_grid.h
@@ -20,6 +20,10 @@ namespace views {
 class Widget;
 }
 
+namespace wm {
+class Shadow;
+}
+
 namespace ash {
 
 class WindowSelectorItem;
@@ -174,6 +178,9 @@ class ASH_EXPORT WindowGrid : public WmWindowObserver {
   // Widget that indicates to the user which is the selected window.
   std::unique_ptr<views::Widget> selection_widget_;
 
+  // Shadow around the selector.
+  std::unique_ptr<::wm::Shadow> selector_shadow_;
+
   // Current selected window position.
   size_t selected_index_;