Revert "GroupSuggestOracle: Remove unused constructor from GroupInfo inner class"

This reverts commit e2e314bd51. Without
this empty protected constructor, cookbook plugin compilation fails with
the following error:

Line 78: Constructors must be 'protected' in subclasses of JavaScriptObject

Change-Id: Ia72bdcf8ccc2eef95ae49138de72c4956d12993e

gerrit-plugin-gwtui/src/main/java/com/google/gerrit/plugin/client/ui/GroupSuggestOracle.java

@@ -77,5 +77,8 @@ public class GroupSuggestOracle extends SuggestOracle {
 
   private static class GroupInfo extends JavaScriptObject {
     // Dummy class, does nothing.
+
+    protected GroupInfo() {
+    }
   }
 }