Add benwells@ to extensions API_OWNERS for Chrome OS apps APIs

Bug: None
Change-Id: I7f4214c2ba46c3e79bed607748e0e3ec045feab8
Reviewed-on: https://chromium-review.googlesource.com/1098745Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567070}

extensions/common/api/API_OWNERS

 # For Extensions and Apps APIs.
-rdevlin.cronin@chromium.org
 lazyboy@chromium.org
+rdevlin.cronin@chromium.org
 
 # For Chrome OS apps APIs.
-tbarzic@chromium.org
+benwells@chromium.org
 stevenjb@chromium.org
+tbarzic@chromium.org