update_engine: add COMPONENT to OWNERS file

update_engine bugs should be filed against Internals>Installer component, add that information to the OWNERS file to make it easier to file bugs
against the correct component. Also add comments showing this COMPONENT
is Chromium OS only since the OWNERS file is shared by Android.

BUG=chromium:977646
TEST=None

Change-Id: Ic918378bb786c9b79790bb155dcf142497fcae17
Reviewed-on: https://chromium-review.googlesource.com/1678727
Tested-by: Xiaochu Liu <xiaochu@chromium.org>
Commit-Ready: Xiaochu Liu <xiaochu@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/OWNERS b/OWNERS
index 07ee38e..6709d7a 100644
--- a/OWNERS
+++ b/OWNERS
@@ -8,3 +8,6 @@
 benchan@google.com
 ahassani@google.com
 xiaochu@google.com
+
+# Chromium OS only:
+# COMPONENT: Internals>Installer