pushChunk(chunk) {
		const oldIdx = this.chunks.indexOf(chunk);
		if (oldIdx >= 0) {
			return false;
		}
		this.chunks.push(chunk);
		return true;
	}
add a chunk into ChunkGroup. Is pushed on or prepended
@param {Chunk} chunk chunk being pushed into ChunkGroupS
@returns {boolean} returns true if chunk addition was successful.