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}

webrtc/modules/video_coding/video_receiver_unittest.cc

@@ -34,8 +34,7 @@ class TestVideoReceiver : public ::testing::Test {
 
   virtual void SetUp() {
     receiver_.reset(new VideoReceiver(&clock_, &event_factory_));
-    EXPECT_EQ(0, receiver_->RegisterExternalDecoder(&decoder_,
-                                                    kUnusedPayloadType, true));
+    receiver_->RegisterExternalDecoder(&decoder_, kUnusedPayloadType, true);
     const size_t kMaxNackListSize = 250;
     const int kMaxPacketAgeToNack = 450;
     receiver_->SetNackSettings(kMaxNackListSize, kMaxPacketAgeToNack, 0);