Bug 657751: don't prune or print blacklist when destroying the GC (r=treilly).
Bug 657751: don't prune or print blacklist when destroying the GC (r=treilly).

Note that I'm not confident about printing during destruction added in
this patch; I'm not sure what invariants the black list system is
assuming about the state of the heap graph when it runs, and whether I
would actually need to e.g. finish any remaining marking work before
invoking printBlacklist.  Still this should make the overall runtime
more robust.  (See Bug 657751, comment 2.)

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