[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/test/frame_generator.cc b/test/frame_generator.cc
index 68abede..8daeb71 100644
--- a/test/frame_generator.cc
+++ b/test/frame_generator.cc
@@ -197,7 +197,7 @@
     RTC_DCHECK_GT(frame_repeat_count, 0);
   }
 
-  virtual ~YuvFileGenerator() {
+  ~YuvFileGenerator() override {
     for (FILE* file : files_)
       fclose(file);
   }
@@ -359,7 +359,7 @@
     RTC_DCHECK_GT(scroll_time_ms + pause_time_ms, 0);
   }
 
-  virtual ~ScrollingImageFrameGenerator() {}
+  ~ScrollingImageFrameGenerator() override {}
 
   VideoFrame* NextFrame() override {
     const int64_t kFrameDisplayTime = scroll_time_ + pause_time_;