Bug 928736 - Remove forward declarations of classes subsequently defined with attributes, because clang doesn't like it when you do this, and add JS:: to the few places that now need it.  r=benjamin

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extra : amend_source : 103b55a665c89b2cc1fddf100b9ec91a8f34efd6
Bug 924696: Expose count of createObjectURL'ed blobs in about:memory. r=njn