Return only visible notifications in FindVisibleNotificationById

I believe we should use |visible_notifications_| instead of
|notification_list_| here, As I checked the CL which introduced
the |visible_notifications_| [1].
[1] https://chromium.googlesource.com/chromium/src/+/0798aac5cb3433e63a605303e4876b7d837e5c7d%5E%21/#F11

Context of this CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1372557/12/ui/message_center/fake_message_center.cc#44

Bug: none
Test: none
Change-Id: I8662eb6e7ea3415dcc8026d031c60afd66b2f036
Reviewed-on: https://chromium-review.googlesource.com/c/1475687
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633178}
1 file changed