Bug 903274: have the search bar binding's initialization callback bail out if the binding is destroyed, r=MattN

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extra : rebase_source : ed745738ebaf085b96d83ba5c6ae270d5dde3b9a
Bug 957902: back out changeset aeef01e3a52a (bug 950405), disabling the temporary whatsnew page for en-US Nightly users