Don't allocate VP8LHashChain, but treat like automatic object

the unique instance of VP8LHashChain (1MB size corresponding to hash_to_first_index_)
is now wholy part of VP8LEncoder, instead of maintaining the pointer to VP8LHashChain
in the encoder.

Change-Id: Ib6fe52019fdd211fbbc78dc0ba731a4af0728677