Adds SpdyStringPieceCaseHash and SpdyStringPieceCaseEq, for future use in SpdyHeaderBlock. SpdyStringPieceCaseHash could be implemented by incorporating Chromium's ToLowerASCII into a function object like StringPieceHashImpl: https://cs.chromium.org/chromium/src/base/strings/string_util.h?l=87&rcl=b493570b9e803c8423c3b9de45a36b6fb82dd970 https://cs.chromium.org/chromium/src/base/strings/string_piece.h?l=532&rcl=9c7be4ec394e834bc5e687c82ccacb379ae26cd3 SpdyStringPieceCaseEq could be implemented using Chromium's EqualsCaseInsensitiveASCII: https://cs.chromium.org/chromium/src/base/strings/string_util.h?l=139&rcl=b493570b9e803c8423c3b9de45a36b6fb82dd970 gfe-relnote: n/a (new code, not yet used in the GFE) PiperOrigin-RevId: 287994767 Change-Id: Ie47ac5e93a7f61d12ceef28abaa1e18e9a62fb06