[win] Add a progress bar in the app launcher for app installs.

ui/app_list/app_list_item_model_observer.h

Index: ui/app_list/app_list_item_model_observer.h
diff --git a/ui/app_list/app_list_item_model_observer.h b/ui/app_list/app_list_item_model_observer.h
index 0e5f99d207aa133e946272a6e2dc5c31dfb1d8f3..80abb3989b59b5c44f9754c2ad998899d8fd5eb3 100644
--- a/ui/app_list/app_list_item_model_observer.h
+++ b/ui/app_list/app_list_item_model_observer.h
@@ -20,6 +20,12 @@ class APP_LIST_EXPORT AppListItemModelObserver {
   // Invoked after item's highlighted state is changed.
   virtual void ItemHighlightedChanged() = 0;
 
+  // Invoked after item begins or finishes installing.
+  virtual void ItemIsInstallingChanged() = 0;
+
+  // Invoked after item's download percentage changes.
+  virtual void ItemPercentDownloadedChanged() = 0;
+
  protected:
   virtual ~AppListItemModelObserver() {}
 };