Change return types of HistogramEnumString from std::string to absl::string_view.

These functions always return string literals, so no need to create a string.

This doesn't have much performance impact on FDO/propeller optimized binaries, but in our non-FDO prototype binary, we are spending 0.2% of CPU just on the temporary `std::string` construction and conversion to `string_view`.

PiperOrigin-RevId: 988265401

quiche/quic/core/quic_types.cc

diff --git a/quiche/quic/core/quic_types.cc b/quiche/quic/core/quic_types.cc
index 2b82db8..dad805a 100644
--- a/quiche/quic/core/quic_types.cc
+++ b/quiche/quic/core/quic_types.cc
@@ -90,7 +90,7 @@
   return os;
 }
 
-std::string HistogramEnumString(WriteStatus enum_value) {
+absl::string_view HistogramEnumString(WriteStatus enum_value) {
   switch (enum_value) {
     case WRITE_STATUS_OK:
       return "OK";

quiche/quic/core/quic_types.h

diff --git a/quiche/quic/core/quic_types.h b/quiche/quic/core/quic_types.h
index e1fe120..2a3c822 100644
--- a/quiche/quic/core/quic_types.h
+++ b/quiche/quic/core/quic_types.h
@@ -118,7 +118,7 @@
   WRITE_STATUS_NUM_VALUES,
 };
 
-std::string HistogramEnumString(WriteStatus enum_value);
+absl::string_view HistogramEnumString(WriteStatus enum_value);
 QUICHE_EXPORT std::ostream& operator<<(std::ostream& os,
                                        const WriteStatus& status);