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}

media/engine/simulcast_encoder_adapter_unittest.cc

@@ -204,10 +204,6 @@ class MockVideoEncoder : public VideoEncoder {
               (FecControllerOverride * fec_controller_override),
               (override));
 
-  // 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.
   int32_t InitEncode(const VideoCodec* codecSettings,
                      const VideoEncoder::Settings& settings) override {
     codec_ = *codecSettings;