Change variable types in BalsaHeaders::ParseTokenList().

QUICHE does not compile in Envoy on Windows after some recent C++20-related
fixes.  This motivated cl/504122725, but that did not fix the issue, because the
C++20-only (begin iterator, count) constructor signature is not available, and
an iterator cannot implicitly be converted to a pointer type on that platform.
The current change reverts cl/504122725 and instead uses
absl::string_view::data() which return absl::string_view::pointer_type instead
of iterator_type.
PiperOrigin-RevId: 505126369

quiche/balsa/balsa_headers.cc

diff --git a/quiche/balsa/balsa_headers.cc b/quiche/balsa/balsa_headers.cc
index 7dfc344..80642c3 100644
--- a/quiche/balsa/balsa_headers.cc
+++ b/quiche/balsa/balsa_headers.cc
@@ -179,8 +179,8 @@
   if (header_value.empty()) {
     return;
   }
-  auto start = header_value.begin();
-  auto end = header_value.end();
+  const char* start = header_value.data();
+  const char* end = header_value.data() + header_value.size();
   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.
-    auto nws = start;
+    const char* nws = start;
 
     // search for next whitspace or separator char.
     while (*start != ',' && static_cast<unsigned char>(*start) > ' ') {