Android: removes obsolete comment.

Following crrev.com/144038, RunTests is executed in the main thread,
so the comment is obsolete.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22890005

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@218032 4ff67af0-8c30-449e-8e8b-ad334ec8d88c