Add extensions_unittests target to ChromiumOS testers.

This should make it run in the ChromiumOS config on the Main Waterfall.

BUG=399315
TEST=None
R=sky@chromium.org

Review URL: https://codereview.chromium.org/437693002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@287465 4ff67af0-8c30-449e-8e8b-ad334ec8d88c