Wallpaper manager: fix calculation for remaining time until next wallpaper update.
Wallpaper manager: fix calculation for remaining time until next wallpaper update.

Define remaining time as time until the next midnight, so that it will not be
negative due to Daylight Saving Time.

BUG=158455
TEST=the following:
(1) manually enable daily wallpaper update (re-enable old picker dialog in about://flags, restart, then enable daily update)
(2) manually set system time to 11/03/2012 0:30AM and 11/04/2012 0:30AM (11/4 is the DST transition but just to make sure it works on both dates)
(3) verify that system does not hang. my test debug log showed that on 11/4/2012 0:30 the remaining time is 24:30 hours.

Review URL: https://codereview.chromium.org/11273101

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@164826 0039d316-1c4b-4281-b951-d872f2087c98

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