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}
diff --git a/rtc_base/string_encode.h b/rtc_base/string_encode.h
index c63d527..87c5bc8 100644
--- a/rtc_base/string_encode.h
+++ b/rtc_base/string_encode.h
@@ -28,7 +28,7 @@
 // String Encoding Utilities
 //////////////////////////////////////////////////////////////////////
 
-std::string hex_encode(const std::string& str);
+std::string hex_encode(absl::string_view str);
 std::string hex_encode(const char* source, size_t srclen);
 std::string hex_encode_with_delimiter(const char* source,
                                       size_t srclen,
@@ -51,10 +51,10 @@
                                  char delimiter);
 
 // Helper functions for hex_decode.
-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);
 size_t hex_decode_with_delimiter(char* buffer,
                                  size_t buflen,
-                                 const std::string& source,
+                                 absl::string_view source,
                                  char delimiter);
 
 // Joins the source vector of strings into a single string, with each
@@ -89,7 +89,7 @@
 std::string ToString(bool b);
 
 std::string ToString(const char* s);
-std::string ToString(std::string t);
+std::string ToString(absl::string_view s);
 
 std::string ToString(short s);
 std::string ToString(unsigned short s);