ash: Fix system tray UAF around user switching

Bug: 965517
Change-Id: Ic059d5b5a0c70701b657d66424d47f5c4de9ae41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623496
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662187}
9 files changed