Add michaelpg to extensions/shell/OWNERS

and correct the component.

BUG=none

Review-Url: https://codereview.chromium.org/2821623002
Cr-Commit-Position: refs/heads/master@{#465733}

extensions/shell/OWNERS

-derat@chromium.org
-jamescook@chromium.org
+michaelpg@chromium.org
+rdevlin.cronin@chromium.org
 rkc@chromium.org
 
-# TEAM: extensions-dev@chromium.org
-# COMPONENT: Platform>Extensions
+# TEAM: apps-dev@chromium.org
+# COMPONENT: Platform>Apps>Shell