mash: Replace int ShelfIDs with AppLaunchID strings.

ash/shelf/shelf_view.h

Index: ash/shelf/shelf_view.h
diff --git a/ash/shelf/shelf_view.h b/ash/shelf/shelf_view.h
index 894b62004b3b19e6246fbb6d34e48d10d4fa8a2a..fcaf1a532a8fd11bcd67dd3bc91fe93d585b3db2 100644
--- a/ash/shelf/shelf_view.h
+++ b/ash/shelf/shelf_view.h
@@ -84,10 +84,10 @@ class ASH_EXPORT ShelfView : public views::View,
   // Returns the ideal bounds of the specified item, or an empty rect if id
   // isn't know. If the item is in an overflow shelf, the overflow icon location
   // will be returned.
-  gfx::Rect GetIdealBoundsOfItemIcon(ShelfID id);
+  gfx::Rect GetIdealBoundsOfItemIcon(const ShelfID& id);
 
   // Repositions the icon for the specified item by the midpoint of the window.
-  void UpdatePanelIconPosition(ShelfID id, const gfx::Point& midpoint);
+  void UpdatePanelIconPosition(const ShelfID& id, const gfx::Point& midpoint);
 
   // Returns true if we're showing a menu.
   bool IsShowingMenu() const;
@@ -395,7 +395,7 @@ class ASH_EXPORT ShelfView : public views::View,
   int start_drag_index_ = -1;
 
   // Used for the context menu of a particular item.
-  ShelfID context_menu_id_ = 0;
+  ShelfID context_menu_id_;
 
   std::unique_ptr<views::FocusSearch> focus_search_;
 
@@ -423,9 +423,8 @@ class ASH_EXPORT ShelfView : public views::View,
   // and it needs to be deleted/unpinned again if the operation gets cancelled.
   bool drag_and_drop_item_pinned_ = false;
 
-  // The ShelfItem which is currently used for a drag and a drop operation
-  // or 0 otherwise.
-  ShelfID drag_and_drop_shelf_id_ = 0;
+  // The ShelfItem currently used for drag and drop; empty if none.
+  ShelfID drag_and_drop_shelf_id_;
 
   // The application ID of the application which we drag and drop.
   std::string drag_and_drop_app_id_;