Add mukai and xiyuan to API owners of chromeos related APIs

ChromeOS related APIs are still actively updated. This is especially
because those APIs (e.g. autotestPrivate) are heavily used by
Tast tests.

Due to the role changes of some of the current owners, I guess
it would be better to have new owners who have experience of
Tast tests.

Bug: none
Test: none
Change-Id: I3b14bc26a38dfb739cbd0cd56bcd0acc96ef1ebc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2142955Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759084}

extensions/common/api/API_OWNERS

@@ -14,6 +14,8 @@ rdevlin.cronin@chromium.org
 karandeepb@chromium.org
 
 # For Chrome OS apps APIs.
-benwells@chromium.org
-stevenjb@chromium.org
 tbarzic@chromium.org
+mukai@chromium.org
+xiyuan@chromium.org
+stevenjb@chromium.org
+benwells@chromium.org