Fix -Wextra-semi warnings.

Starting from https://chromium-review.googlesource.com/c/1485012,
-Wextra-semi is enabled and WebRTC has some violations to fix.

This is a follow-up of https://webrtc-review.googlesource.com/c/123560.

Bug: webrtc:10355
Change-Id: I012b7497fc8991037fd77aa98f1579c22e08206f
Reviewed-on: https://webrtc-review.googlesource.com/c/124126
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26831}

modules/video_coding/generic_encoder_unittest.cc

@@ -42,7 +42,7 @@ class FakeEncodedImageCallback : public EncodedImageCallback {
         encoded_image.timing_.flags != VideoSendTiming::kNotTriggered;
     last_capture_timestamp_ = encoded_image.capture_time_ms_;
     return Result(Result::OK);
-  };
+  }
 
   void OnDroppedFrame(DropReason reason) override { ++num_frames_dropped_; }