When using "restore defaults", Customize mode unwraps all elements. The previous implementation
of _updateNewTabVisibility assumed that elements were wrapped in <toolbarpaletteitem>s when in
customize mode, but this isn't true during a reset. The new implementation just looks for the
<toolbarpaletteitem>s and their children as necessary.
MozReview-Commit-ID: FjNJ1n8foGi
--HG--
extra : rebase_source : c9de8a2a4d4dcf191281fead320acc2ddc1321d6
