Automated checkin: version bump for firefox 21.0 release. DONTBUILD CLOSED TREE a=release

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branch : GECKO210_2013051217_RELBRANCH
Added FIREFOX_21_0_RELEASE FIREFOX_21_0_BUILD4 tag(s) for changeset 916fdce8831c. DONTBUILD CLOSED TREE a=release

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branch : GECKO210_2013051217_RELBRANCH