Use ChromeLauncherControllerImpl::GetItem; cleanup.

ash/common/shelf/shelf_model.cc

Index: ash/common/shelf/shelf_model.cc
diff --git a/ash/common/shelf/shelf_model.cc b/ash/common/shelf/shelf_model.cc
index 04705e42bba10079bad171eca6d54d0ad960a08c..e5254194943251de107a35898f92308d4c35c6c1 100644
--- a/ash/common/shelf/shelf_model.cc
+++ b/ash/common/shelf/shelf_model.cc
@@ -90,7 +90,11 @@ void ShelfModel::Move(int index, int target_index) {
 }
 
 void ShelfModel::Set(int index, const ShelfItem& item) {
-  DCHECK(index >= 0 && index < item_count());
+  if (index < 0 || index >= item_count()) {
+    NOTREACHED();
+    return;
+  }
+
   int new_index = item.type == items_[index].type
                       ? index
                       : ValidateInsertionIndex(item.type, index);