Adopt absl::string_view in rtc_base/log_sinks

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

This CL adds absl::string_view versions of the OnLogMessage functions in
rtc::LogSink. The const std::string& versions are kept for now since
downstream clients use subclasses of LogSink and need to be migrated
before these are removed.

Bug: webrtc:13579
Change-Id: I57bb72ad503805ff0ca16f1d7aece2d44c65cb73
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/253980
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Xavier Lepaul‎ <xalep@webrtc.org>
Owners-Override: Xavier Lepaul‎ <xalep@webrtc.org>
Commit-Queue: Ali Tofigh <alito@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36189}

sdk/objc/helpers/NSString+StdString.h

@@ -12,6 +12,8 @@
 
 #include <string>
 
+#include "absl/strings/string_view.h"
+
 NS_ASSUME_NONNULL_BEGIN
 
 @interface NSString (StdString)
@@ -23,4 +25,10 @@ NS_ASSUME_NONNULL_BEGIN
 
 @end
 
+@interface NSString (AbslStringView)
+
++ (NSString *)stringForAbslStringView:(const absl::string_view)abslStringView;
+
+@end
+
 NS_ASSUME_NONNULL_END