Adopt absl::string_view in function parameters under rtc_base/

This is part of a large-scale effort to increase adoption of
absl::string_view across the WebRTC code base.

This CL converts the majority of "const std::string&"s in function
parameters under rtc_base/ to absl::string_view.

Bug: webrtc:13579
Change-Id: I2b1e3776aa42326aa405f76bb324a2d233b21dca
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/254081
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Xavier Lepaul‎ <xalep@webrtc.org>
Reviewed-by: Anders Lilienthal <andersc@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Ali Tofigh <alito@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36239}

rtc_base/string_encode.cc

@@ -12,6 +12,7 @@
 
 #include <cstdio>
 
+#include "absl/strings/string_view.h"
 #include "rtc_base/arraysize.h"
 #include "rtc_base/checks.h"
 
@@ -77,8 +78,8 @@ void hex_encode_with_delimiter(char* buffer,
 
 }  // namespace
 
-std::string hex_encode(const std::string& str) {
-  return hex_encode(str.c_str(), str.size());
+std::string hex_encode(absl::string_view str) {
+  return hex_encode(str.data(), str.size());
 }
 
 std::string hex_encode(const char* source, size_t srclen) {
@@ -141,14 +142,14 @@ size_t hex_decode_with_delimiter(char* cbuffer,
   return bufpos;
 }
 
-size_t hex_decode(char* buffer, size_t buflen, const std::string& source) {
+size_t hex_decode(char* buffer, size_t buflen, absl::string_view source) {
   return hex_decode_with_delimiter(buffer, buflen, source, 0);
 }
 size_t hex_decode_with_delimiter(char* buffer,
                                  size_t buflen,
-                                 const std::string& source,
+                                 absl::string_view source,
                                  char delimiter) {
-  return hex_decode_with_delimiter(buffer, buflen, source.c_str(),
+  return hex_decode_with_delimiter(buffer, buflen, source.data(),
                                    source.length(), delimiter);
 }
 
@@ -177,7 +178,7 @@ bool tokenize_first(absl::string_view source,
                     std::string* rest) {
   // Find the first delimiter
   size_t left_pos = source.find(delimiter);
-  if (left_pos == std::string::npos) {
+  if (left_pos == absl::string_view::npos) {
     return false;
   }
 
@@ -245,8 +246,9 @@ std::string ToString(const bool b) {
 std::string ToString(const char* const s) {
   return std::string(s);
 }
-std::string ToString(const std::string s) {
-  return s;
+
+std::string ToString(absl::string_view s) {
+  return std::string(s);
 }
 
 std::string ToString(const short s) {