Fixing problem introduced in one of my previous commits.
Changing fc->tx_probs back to fc->pre_tx_probs. This change actually affects the bitstream but current test vectors work. Chrome branch is not affected at all. Broken since: cc662dd7 Adding struct tx_probs and struct tx_counts to cleanup the code. Change-Id: I36dd4b3678e902e10aba8dd49b0012eb558c209d