Deprecate VideoDecoder::Reset() and remove calls.

Removes calls to decoder reset and instead drops delta frames and
requests keyframes until one arrives.

BUG=webrtc:5475
R=stefan@webrtc.org
TBR=mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/1647163002 .

Cr-Commit-Position: refs/heads/master@{#11460}

webrtc/modules/video_coding/include/mock/mock_video_codec_interface.h

@@ -72,7 +72,6 @@ class MockVideoDecoder : public VideoDecoder {
   MOCK_METHOD1(RegisterDecodeCompleteCallback,
                int32_t(DecodedImageCallback* callback));
   MOCK_METHOD0(Release, int32_t());
-  MOCK_METHOD0(Reset, int32_t());
   MOCK_METHOD0(Copy, VideoDecoder*());
 };