Bug 921224 (part 1) - Don't use ballast in MarkStack.  r=wmccloskey.

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extra : rebase_source : f847b35b7d5879816f1cde112c1f3cbf0fe58414
Bug 921224 (part 2) - Use a smaller mark stack when incremental GC is disabled.  r=wmccloskey.

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extra : rebase_source : 86fdb1f981db31751f07a1e2c367caa31637cc5a