[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}

modules/video_coding/video_coding_impl.cc

@@ -50,7 +50,7 @@ class VideoCodingModuleImpl : public VideoCodingModule {
         timing_(new VCMTiming(clock)),
         receiver_(clock, timing_.get(), nack_sender, keyframe_request_sender) {}
 
-  virtual ~VideoCodingModuleImpl() {}
+  ~VideoCodingModuleImpl() override {}
 
   int64_t TimeUntilNextProcess() override {
     int64_t receiver_time = receiver_.TimeUntilNextProcess();