Add basic support for launching mash shelf pinned apps.

mash/shelf/public/interfaces/shelf.mojom

Index: mash/shelf/public/interfaces/shelf.mojom
diff --git a/mash/shelf/public/interfaces/shelf.mojom b/mash/shelf/public/interfaces/shelf.mojom
index 5465f574a2530b7d7f3f74eea65669b280cf0165..077900260d27d46894f22f8b0deb5fa0e971e719 100644
--- a/mash/shelf/public/interfaces/shelf.mojom
+++ b/mash/shelf/public/interfaces/shelf.mojom
@@ -30,6 +30,9 @@ interface ShelfObserver {
 
 // ShelfItemDelegate handles command execution and observes shelf item changes.
 interface ShelfItemDelegate {
+  // Called when a pinned shelf item is invoked without an open window.
+  LaunchItem();
+
   // Called on invocation of a shelf item's context menu command.
   ExecuteCommand(uint32 command_id, int32 event_flags);