Moved FolderImageSource class to a separate file.

ui/app_list/app_list_folder_item.h

Index: ui/app_list/app_list_folder_item.h
diff --git a/ui/app_list/app_list_folder_item.h b/ui/app_list/app_list_folder_item.h
index 3610fc34ca032e4fcccd3e916ca2ede023eec6ca..cd572e71dbba2c43bfad6f7f407c49d72efef12c 100644
--- a/ui/app_list/app_list_folder_item.h
+++ b/ui/app_list/app_list_folder_item.h
@@ -68,12 +68,6 @@ class APP_LIST_EXPORT AppListFolderItem : public AppListItem,
   void OnExtensionPreferenceChanged() override;
   bool CompareForTest(const AppListItem* other) const override;
 
-  // Calculates the top item icons' bounds inside |folder_icon_bounds|.
-  // Returns the bounds of top item icons in sequence of top left, top right,
-  // bottom left, bottom right.
-  static std::vector<gfx::Rect> GetTopIconsBounds(
-      const gfx::Rect& folder_icon_bounds);
-
   // Returns an id for a new folder.
   static std::string GenerateId();