Fix override declarations and delete related TODOs

Bug: webrtc:10198, chromium:428099
Change-Id: Ic7b0dd3c58c3daa5ade4d2c503b77a51b29c716e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/179380
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31739}

test/mock_audio_encoder.h

@@ -21,10 +21,6 @@ namespace webrtc {
 
 class MockAudioEncoder : public AudioEncoder {
  public:
-  // TODO(nisse): Valid overrides commented out, because the gmock
-  // methods don't use any override declarations, and we want to avoid
-  // warnings from -Winconsistent-missing-override. See
-  // http://crbug.com/428099.
   MockAudioEncoder();
   ~MockAudioEncoder();
   MOCK_METHOD(int, SampleRateHz, (), (const, override));