Adopt absl::string_view in rtc_base/ (straightforward cases)

Bug: webrtc:13579
Change-Id: I240db6285abb22652242bc0b2ebe9844ec4a45f0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/258723
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Ali Tofigh <alito@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36561}

rtc_base/ssl_stream_adapter_unittest.cc

@@ -685,7 +685,7 @@ class SSLStreamAdapterTestBase : public ::testing::Test,
       return server_ssl_->GetSslVersion();
   }
 
-  bool ExportKeyingMaterial(const char* label,
+  bool ExportKeyingMaterial(absl::string_view label,
                             const unsigned char* context,
                             size_t context_len,
                             bool use_context,