When the change view is visible, it sets a timer to check whether the
loaded patch set is the latest. If there is a newer patch, a toast alert
is shown to recommend reloading the change to view the latest revision.
A newer patch has been uploaded. [Reload]
When the change view is unloaded, or the tab is not in the foreground,
the timer is canceled.
Feature: Issue 5875
Change-Id: I1c52242164002d05e08f06b9790d6b3177bca9b9