hg: jdk/sandbox: Fix broken generated Event::commit() and add test to verify behavior

[erik.gahlin at oracle.com]

Changeset: 7758008806d1
Author:    egahlin
Date:      2019-08-16 20:01 +0200
URL:       https://hg.openjdk.java.net/jdk/sandbox/rev/7758008806d1

Fix broken generated Event::commit() and add test to verify behavior

! src/jdk.jfr/share/classes/jdk/jfr/internal/EventInstrumentation.java
+ test/jdk/jdk/jfr/api/event/TestEventDuration.java