Add jming and dtrainor as OWNERS for download Android files.

Bug: 
Change-Id: I2a724e96e1fc180d1a17d358a3a3b0b602a9f154
Reviewed-on: https://chromium-review.googlesource.com/802139Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Joy Ming <jming@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520750}

chrome/android/java/src/org/chromium/chrome/browser/download/OWNERS

+dtrainor@chromium.org
+jming@chromium.org
 qinmin@chromium.org
 
 per-file DownloadActivity.java=twellington@chromium.org