[ESLint] Fix violations when enabling curly rule in eslint.

tracing/tracing/model/proxy_selectable_item.html

Index: tracing/tracing/model/proxy_selectable_item.html
diff --git a/tracing/tracing/model/proxy_selectable_item.html b/tracing/tracing/model/proxy_selectable_item.html
index 7e2d91ab6b889694cd161135fb2feab8d6e5ed01..6ed07d72f351efc53d2983770ea69b9ee4e9a14a 100644
--- a/tracing/tracing/model/proxy_selectable_item.html
+++ b/tracing/tracing/model/proxy_selectable_item.html
@@ -31,8 +31,9 @@ tr.exportTo('tr.model', function() {
 
     get selectionState() {
       var modelItem = this.modelItem_;
-      if (modelItem === undefined)
+      if (modelItem === undefined) {
         return SelectionState.NONE;
+      }
       return modelItem.selectionState;
     }
   };