Uses uint8_t for an array index value.

Some embedders of QUICHE use a signed char type.

PiperOrigin-RevId: 446449853

quiche/http2/adapter/header_validator.cc

diff --git a/quiche/http2/adapter/header_validator.cc b/quiche/http2/adapter/header_validator.cc
index cb3edf9..cfd14f1 100644
--- a/quiche/http2/adapter/header_validator.cc
+++ b/quiche/http2/adapter/header_validator.cc
@@ -43,7 +43,7 @@
 CharMap AllowObsText(CharMap map) {
   // Characters above 0x80 are allowed in header field values as `obs-text` in
   // RFC 7230.
-  for (char c = 0xff; c >= 0x80; --c) {
+  for (uint8_t c = 0xff; c >= 0x80; --c) {
     map[c] = true;
   }
   return map;