Optmize VP8LGetBackwardReferences LZ77 references.

Use the refs_lz77 computed (with cache_bits=0) in the method 'CalculateBestCacheSize'
to regenerate the LZ77 references corresponding to the optimum cache_bits and avoid
calling costly 'BackwardReferencesLz77' one extra time.

This change leaves the compression density unchanged and speeds up compression
by 10-15%.

Change-Id: I5a92e11788d3c3f656aa7e1fba54fb5d96ee0027