Add Sydney team to headless owners

Bug: 
Change-Id: I7adc36e09d570d3034c2882817489bacdb84b493
Reviewed-on: https://chromium-review.googlesource.com/562911
Commit-Queue: David Vallet <dvallet@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485180}

headless/OWNERS

@@ -2,6 +2,10 @@ skyostil@chromium.org
 alexclarke@chromium.org
 altimin@chromium.org
 eseckler@chromium.org
+dvallet@chromium.org
+irisu@chromium.org
+jzfeng@chromium.org
+rvera@chromium.org
 
 # TEAM: headless-dev@chromium.org
 # COMPONENT: Internals>Headless