Reserves the right amount of space for a set in BalsaHeaders::RemoveAllOfHeaderInList().

Protected by unrisky change, not protected.

PiperOrigin-RevId: 627493216
diff --git a/quiche/balsa/balsa_headers.cc b/quiche/balsa/balsa_headers.cc
index 6028a98..1747609 100644
--- a/quiche/balsa/balsa_headers.cc
+++ b/quiche/balsa/balsa_headers.cc
@@ -627,6 +627,7 @@
   // Better performance can be achieved by asking caller to lower case
   // the keys and RemoveAllOfheaderInlist just does lookup.
   absl::flat_hash_set<std::string> lowercase_keys;
+  lowercase_keys.reserve(keys.size());
   for (const auto& key : keys) {
     MaybeClearSpecialHeaderValues(key);
     lowercase_keys.insert(absl::AsciiStrToLower(key));