Rename SpdyHeaderBlock to Http2HeaderBlock. Salvaged from a botched Zwingli attempt: http://refactor/16032281396283088571 created by go/Zwingli Tested: TAP train for global presubmit queue http://test/OCL:338336207:BASE:338335676:1603342555112:bf8a0bd2 PiperOrigin-RevId: 338549254 Change-Id: I30218655533383493f2ad971e2cb27f0983d27a9