update_engine: edit OWNERS

Add xiaochu@google.com to OWNERS so related party can be held
responsible for Chrome OS changes regarding DLC or the fuzzers.

BUG=chromium:906815
TEST=None

Change-Id: I7769ca704e3d706d14565d9b388e09852d2407db
Reviewed-on: https://chromium-review.googlesource.com/1344913
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/OWNERS b/OWNERS
index 0bf7587..07ee38e 100644
--- a/OWNERS
+++ b/OWNERS
@@ -7,3 +7,4 @@
 # Chromium OS maintainers:
 benchan@google.com
 ahassani@google.com
+xiaochu@google.com