[Media Notification] Handle null session info

Since ash::MediaNotificationItem is a media controller
we should be able to handle a null media session info.
This fixes a segfault caused by not handling this
scenario.

BUG=897836

Change-Id: I7869e14de9fd3d9d094f21d3daa5ff3260c3e3f8
Reviewed-on: https://chromium-review.googlesource.com/c/1457198
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630373}
3 files changed