This change simplifies SpdyHeaderBlock and SpdyHeaderStorage, now that the underlying arena implementation does not allocate memory upon construction. gfe-relnote: Code simplifications to classes in //third_party/spdy, no functional change; not protected. PiperOrigin-RevId: 284805587 Change-Id: I3b0cbdda821c1f2ea89b9f880d76a3132490b196