Return `const char*` from ToString(RTCErrorType error).

Returning absl::string_view causes problems to the Chromium/WebRTC
component build because absl::operator<< needs to be exported.

This CL switches to `const char*` which should be enough to avoid
to generate temporaries.

Bug: webrtc:9419
Change-Id: If169a6f95c7efd21ac8ce108c7f2f80a76ff2313
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153842
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29250}
diff --git a/api/rtc_error.cc b/api/rtc_error.cc
index 51fd07f..c9ad7cb 100644
--- a/api/rtc_error.cc
+++ b/api/rtc_error.cc
@@ -10,12 +10,11 @@
 
 #include "api/rtc_error.h"
 
-#include "absl/strings/string_view.h"
 #include "rtc_base/arraysize.h"
 
 namespace {
 
-const absl::string_view kRTCErrorTypeNames[] = {
+const char* kRTCErrorTypeNames[] = {
     "NONE",
     "UNSUPPORTED_OPERATION",
     "UNSUPPORTED_PARAMETER",
@@ -53,7 +52,7 @@
   message_ = std::move(message);
 }
 
-absl::string_view ToString(RTCErrorType error) {
+const char* ToString(RTCErrorType error) {
   int index = static_cast<int>(error);
   return kRTCErrorTypeNames[index];
 }