Background Updates: Record plugin & theme update statistics like we do for core updates.

Pass plugin/theme update objects into the Background updater for consistency with core & translations.

Merges [27905] to the 3.8 branch.

props dd32.
fixes #27633.

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git-svn-id: http://core.svn.wordpress.org/branches/3.8@27753 1a063a9b-81f0-0310-95a4-ce76da25c4cd