Remove faulty DCHECK in rtcp::TransportFeedback DecodeSymbol

BUG=chromium:642745
R=sprang@webrtc.org

Review URL: https://codereview.webrtc.org/2296953003 .

Cr-Commit-Position: refs/heads/master@{#14003}

webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.cc

@@ -60,7 +60,11 @@ TransportFeedback::StatusSymbol DecodeSymbol(uint8_t value) {
       return TransportFeedback::StatusSymbol::kReceivedSmallDelta;
     case 2:
       return TransportFeedback::StatusSymbol::kReceivedLargeDelta;
+    case 3:
+      // It is invalid, but |value| comes from network, so can be any.
+      return TransportFeedback::StatusSymbol::kNotReceived;
     default:
+      // Caller should pass 2 bits max.
       RTC_NOTREACHED();
       return TransportFeedback::StatusSymbol::kNotReceived;
   }