Add media_app_ui/OWNERS as OWNERS for chorme_media_app* in web_applications/

Change-Id: I4daf247e0136bf5a5e95796e0463540099a7b039
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533216Reviewed-by: David Lei <dlei@google.com>
Reviewed-by: dstockwell <dstockwell@chromium.org>
Commit-Queue: dstockwell <dstockwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826593}

chrome/browser/chromeos/web_applications/OWNERS

@@ -7,6 +7,7 @@ per-file chrome_camera_app*=file://chromeos/components/camera_app_ui/OWNERS
 per-file chrome_help_app*=carpenterr@chromium.org
 per-file chrome_file_manager*=file://chromeos/components/file_manager/OWNERS
 per-file file_manager*=file://chromeos/components/file_manager/OWNERS
+per-file chrome_media_app*=file://chromeos/components/media_app_ui/OWNERS
 per-file terminal_source*=calamity@chromium.org
 per-file terminal_source*=joelhockey@chromium.org
 per-file default_web_app_ids.h=jshikaram@chromium.org