This commit produces a bit-exact bitstream because prior calls to code
 the tx_size using aom_write_tree() would automatically convert the
 aom_tree_index on the fly to a CDF.
The tx_size_cdf[]'s are now computed once per frame.
Based on recent tests, this patch reduces decode time by 4%.

Change-Id: I920dbf036b8a9574ea6e65ae5f9c43eef1c3f864