Add mash shelf application id support.

mash/wm/public/interfaces/user_window_controller.mojom

Index: mash/wm/public/interfaces/user_window_controller.mojom
diff --git a/mash/wm/public/interfaces/user_window_controller.mojom b/mash/wm/public/interfaces/user_window_controller.mojom
index 3d84fb11d06376c484484524f6ca499df7c2374e..79dc99fe7641b89da017d3e8db4d2c79f1465420 100644
--- a/mash/wm/public/interfaces/user_window_controller.mojom
+++ b/mash/wm/public/interfaces/user_window_controller.mojom
@@ -9,6 +9,7 @@ struct UserWindow {
   string window_title;
   bool window_has_focus;
   array<uint8> window_app_icon;  // Serialized SkBitmap.
+  string window_app_id;

[sky, 2016/04/22 15:51:19]

Document this. And while you're at it, document window_id (it's not the same as
the internal id used the ws).

[msw, 2016/04/22 19:02:17]

Done.

 };
 
 // An observer of user windows within mojom::Container::USER_WINDOWS.
@@ -21,7 +22,7 @@ interface UserWindowObserver {
   OnUserWindowRemoved(uint32 window_id);
 
   OnUserWindowTitleChanged(uint32 window_id, string window_title);
-  
+
   // |app_icon| is a serialized SkBitmap.
   OnUserWindowAppIconChanged(uint32 window_id, array<uint8> app_icon);