Fix a crash in TrayRotationLock::OnRotationLockChanged
When the StatusAreaWidget is being shutdown, tray views are deleted before their
owning TrayImageItems.
During this process updates to the rotation lock can cause TrayRotationLock to
attempt to access the now destroyed tray view.
Updated TrayRotationLock to exit OnRotationLockChanged early if there is no tray
view.
TEST=TrayRotationLockTest.LockUpdatedDuringDesctruction
BUG=516498
Review URL: https://codereview.chromium.org/
1281903004
Cr-Commit-Position: refs/heads/master@{#343386}