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This CL changes NotificationPromo's HandleViewed/HandleClosed methods to
not update the current instance variables and just save the updated
values to prefs. This is needed because HandleViewed can be called
in the middle of the chain of CanShow calls made during layout of the
NTP, causing some CanShow calls to return true while others to return
false during the same layout. By not saving the updated values to the
current instance variables, the CanShow calls will remain consistent
during the same layout. To facilitate this change, this CL also changes
the return type of HandleViewed to void since the return value is never
used.

BUG=625192

Review-Url: https://codereview.chromium.org/2140173009
Cr-Commit-Position: refs/heads/master@{#407218}