Remove RegisterExternal{De,En}coder error codes.

Also adds a RTC_CHECK in VideoReceiveStream that verifies that decoders
aren't null, since this will attempt to deregister a codec which would
previously fail with an obscure stack trace not indicating what actually
was wrong.

BUG=webrtc:5249
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1479793002 .

Cr-Commit-Position: refs/heads/master@{#10821}
diff --git a/webrtc/modules/video_coding/video_coding_impl.h b/webrtc/modules/video_coding/video_coding_impl.h
index 38fe9ed..cfa0f92 100644
--- a/webrtc/modules/video_coding/video_coding_impl.h
+++ b/webrtc/modules/video_coding/video_coding_impl.h
@@ -82,9 +82,9 @@
   // Same as SendCodecBlocking.  Try to use GetSendCodec() instead.
   VideoCodecType SendCodecBlocking() const;
 
-  int32_t RegisterExternalEncoder(VideoEncoder* externalEncoder,
-                                  uint8_t payloadType,
-                                  bool internalSource);
+  void RegisterExternalEncoder(VideoEncoder* externalEncoder,
+                               uint8_t payloadType,
+                               bool internalSource);
 
   int Bitrate(unsigned int* bitrate) const;
   int FrameRate(unsigned int* framerate) const;
@@ -150,9 +150,9 @@
                                int32_t numberOfCores,
                                bool requireKeyFrame);
 
-  int32_t RegisterExternalDecoder(VideoDecoder* externalDecoder,
-                                  uint8_t payloadType,
-                                  bool internalRenderTiming);
+  void RegisterExternalDecoder(VideoDecoder* externalDecoder,
+                               uint8_t payloadType,
+                               bool internalRenderTiming);
   int32_t RegisterReceiveCallback(VCMReceiveCallback* receiveCallback);
   int32_t RegisterReceiveStatisticsCallback(
       VCMReceiveStatisticsCallback* receiveStats);