Originally we need to send the refresh flag and the virtual indices
mapping for the reference frame buffer update for show_existing_frame to
have the BWDREF_FRAME replace the LAST_FRAME.

To remove sending this information, we update the the virtual indices
of the reference frame buffer after the last_bipred_frame is encoded,
and therefore the decoder will receive the updated reference mapping
at the next non-show-existing frame.

As a result, we can save 4 bytes per show-existing frame, and get 0.12,
0.2, and 0.07 BDRATE improvement in lowres, derf, and midref test set
respectively.

Change-Id: I63d41ee6ea99884798f0778b789d2701e2f2d3e0