Store SubmitTypeRecord lazily in ChangeData

Add convenience method isOk() to SubmitTypeRecord to make it easier
for callers to check whether this result was successful.

Change-Id: I1551d5fdce208631f4d97d815fa882ff44502467

gerrit-common/src/main/java/com/google/gerrit/common/data/SubmitTypeRecord.java

@@ -56,6 +56,10 @@ public class SubmitTypeRecord {
     this.errorMessage = errorMessage;
   }
 
+  public boolean isOk() {
+    return status == Status.OK;
+  }
+
   @Override
   public String toString() {
     StringBuilder sb = new StringBuilder();