Do not convert string_view::iterator to char* in BalsaHeader::ParseTokenList().

This conversion does not seem to work on certain platforms that Envoy needs to
be compiled for.

PiperOrigin-RevId: 504122725

quiche/balsa/balsa_headers.cc

diff --git a/quiche/balsa/balsa_headers.cc b/quiche/balsa/balsa_headers.cc
index 933d631..7dfc344 100644
--- a/quiche/balsa/balsa_headers.cc
+++ b/quiche/balsa/balsa_headers.cc
@@ -179,8 +179,8 @@
   if (header_value.empty()) {
     return;
   }
-  const char* start = header_value.begin();
-  const char* end = header_value.end();
+  auto start = header_value.begin();
+  auto end = header_value.end();
   while (true) {
     // Cast `*start` to unsigned char to make values above 127 rank as expected
     // on platforms with signed char, where such values are represented as
@@ -194,7 +194,7 @@
       }
     }
     // found. marked.
-    const char* nws = start;
+    auto nws = start;
 
     // search for next whitspace or separator char.
     while (*start != ',' && static_cast<unsigned char>(*start) > ' ') {