[clang-tidy] Apply modernize-use-override fixes.

This CL applies clang-tidy's modernize-use-override [1] to 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/modernize-use-override.html

Bug: webrtc:10252
Change-Id: I2bb8bd90fa8adb90aa33861fe7c788132a819a20
Reviewed-on: https://webrtc-review.googlesource.com/c/120412
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26461}
diff --git a/audio/channel_send.cc b/audio/channel_send.cc
index 7899cf7..c16903f 100644
--- a/audio/channel_send.cc
+++ b/audio/channel_send.cc
@@ -410,7 +410,7 @@
  public:
   explicit VoERtcpObserver(ChannelSend* owner)
       : owner_(owner), bandwidth_observer_(nullptr) {}
-  virtual ~VoERtcpObserver() {}
+  ~VoERtcpObserver() override {}
 
   void SetBandwidthObserver(RtcpBandwidthObserver* bandwidth_observer) {
     rtc::CritScope lock(&crit_);