[clang-tidy] Apply performance-for-range-copy fixes.

This CL applies clang-tidy's performance-for-range-copy [1] on the
WebRTC codebase.

All changes in this CL are automatically generated by both clang-tidy
and 'git cl format'.

[1] - https://clang.llvm.org/extra/clang-tidy/checks/performance-for-range-copy.html

Bug: webrtc:10215
Change-Id: I7c83290b8866d76129bbec4e24e6701f5014102e
Reviewed-on: https://webrtc-review.googlesource.com/c/120043
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26420}

pc/sdp_serializer.cc

@@ -81,7 +81,7 @@ rtc::StringBuilder& operator<<(
 rtc::StringBuilder& operator<<(rtc::StringBuilder& builder,
                                const SimulcastLayerList& simulcast_layers) {
   bool first = true;
-  for (auto alternatives : simulcast_layers) {
+  for (const auto& alternatives : simulcast_layers) {
     if (!first) {
       builder << kDelimiterSemicolon;
     }