For the experiment of EXT_REFS, removed the previous special handling
on the new last 3 references, i.e. LAST2_FRAME, LAST3_FRAME, and
LAST4_FRAME, at the decoder, so that these new last references are
treated the same way as the other 3 references (LAST_FRAME,
GOLDEN_FRAME, and ALTREF_FRAME). Encoder changes have been made
accordingly to realize this flexibility.

Change-Id: Ic6546f9443b4377bb7e7b101bfa3e70a8b8d1c65