ash: Add GetItemIndexForType() function to ShelfModel.

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 600c77e007f3b8e7085fa7410573cbb06ea15ef6..36a0b9e3f36debd9cc2e27d34aef7a4177a933c3 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -42,7 +42,6 @@
 #include "ash/shelf/app_list_shelf_item_delegate.h"
 #include "ash/shelf/shelf_layout_manager.h"
 #include "ash/shelf/shelf_model.h"
-#include "ash/shelf/shelf_model_util.h"
 #include "ash/shelf/shelf_widget.h"
 #include "ash/shell_delegate.h"
 #include "ash/shell_factory.h"
@@ -499,7 +498,7 @@ LauncherDelegate* Shell::GetLauncherDelegate() {
 
     // Finding the shelf model's location of the app list and setting its
     // LauncherItemDelegate.
-    int app_list_index = GetShelfItemIndexForType(TYPE_APP_LIST, *shelf_model_);
+    int app_list_index = shelf_model_->GetItemIndexForType(TYPE_APP_LIST);
     DCHECK_GE(app_list_index, 0);
     LauncherID app_list_id = shelf_model_->items()[app_list_index].id;
     DCHECK(app_list_id);