This seems to be a compatibility issue in Eclipse 4.x. I can already reproduce the problem in a very simple example. I reported this as [a bug] you can vote on this bug if you like.
EDIT: Eclipse committer Paul Webster confirms that this doesn't work yet. It may be possible to work around this by using instead of and extending WorkbenchWindowControlContribution instead of ContributionItem.
EDIT: SlowStrider's bug (427475) was closed as duplicate of bug 392457; so if you want to vote on this issue, vote here: https://bugs.eclipse.org/bugs/show_bug.cgi?id=392457. The older bug (392457) is fairly old and has only one vote. (M.)