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/include/video_coding.h b/webrtc/modules/video_coding/include/video_coding.h
index f19475c..5f5961c 100644
--- a/webrtc/modules/video_coding/include/video_coding.h
+++ b/webrtc/modules/video_coding/include/video_coding.h
@@ -181,6 +181,7 @@
     //
     // Return value      : VCM_OK, on success.
     //                     < 0,    on error.
+    // TODO(pbos): Remove return type when unused elsewhere.
     virtual int32_t RegisterExternalEncoder(VideoEncoder* externalEncoder,
                                             uint8_t payloadType,
                                             bool internalSource = false) = 0;
@@ -334,12 +335,9 @@
     //                                 registered to.
     //      - internalRenderTiming   : True if the internal renderer (if any) of the decoder
     //                                 object can make sure to render at a given time in ms.
-    //
-    // Return value      : VCM_OK, on success.
-    //                     < 0,    on error.
-    virtual int32_t RegisterExternalDecoder(VideoDecoder* externalDecoder,
-                                            uint8_t payloadType,
-                                            bool internalRenderTiming) = 0;
+    virtual void RegisterExternalDecoder(VideoDecoder* externalDecoder,
+                                         uint8_t payloadType,
+                                         bool internalRenderTiming) = 0;
 
     // Register a receive callback. Will be called whenever there is a new frame ready
     // for rendering.