Merge "Implement ver4 dictionary GC."

.../policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp

...st/policyimpl/dictionary/bigram/ver4_bigram_list_policy.h

...olicyimpl/dictionary/structure/pt_common/pt_node_params.h

...structure/v3/dynamic_patricia_trie_gc_event_listeners.cpp

...ry/structure/v3/dynamic_patricia_trie_updating_helper.cpp

...l/dictionary/structure/v4/content/bigram_dict_content.cpp

...ictionary/structure/v4/content/probability_dict_content.h

...ionary/structure/v4/content/sparse_table_dict_content.cpp

...ctionary/structure/v4/content/sparse_table_dict_content.h

...ary/structure/v4/content/terminal_position_lookup_table.h

...st/policyimpl/dictionary/structure/v4/ver4_dict_buffers.h

...olicyimpl/dictionary/structure/v4/ver4_dict_constants.cpp

.../policyimpl/dictionary/structure/v4/ver4_dict_constants.h

...ictionary/structure/v4/ver4_patricia_trie_node_writer.cpp

...mpl/dictionary/structure/v4/ver4_patricia_trie_policy.cpp

...ionary/structure/v4/ver4_patricia_trie_writing_helper.cpp

...cyimpl/dictionary/utils/buffer_with_extendable_buffer.cpp

...licyimpl/dictionary/utils/buffer_with_extendable_buffer.h

.../android/inputmethod/latin/Ver4BinaryDictionaryTests.java