Rename SpdyHeaderBlock to Http2HeaderBlock. Salvaged from a botched Zwingli attempt: http://refactor/16032281396283088571 created by go/Zwingli Tested: TAP for global presubmit queue passed after automated deflaking of failures: http://mondo/deflaker/run/9a9c8d4e-50d0-47e0-970b-4b6c24ace2d7 http://mondo/deflaker/run/32a00ba5-16fa-4c78-9e10-afa7c56abc61 http://test/OCL:338336205:BASE:339106963:1603774283861:13fcf12c PiperOrigin-RevId: 339252455 Change-Id: Icce13789ba4b6e74894b0715ec897fc61aba9993