Allow parsing empty RTCP TargetBitrate messages, but stop sending them.

webrtc/common_types.h

Index: webrtc/common_types.h
diff --git a/webrtc/common_types.h b/webrtc/common_types.h
index 73a1c8340d62e93b65b3d0f4894583826c336705..f22e0f452baa06ba6720aac211bdd3390634807d 100644
--- a/webrtc/common_types.h
+++ b/webrtc/common_types.h
@@ -632,6 +632,10 @@ class BitrateAllocation {
     return !(*this == other);
   }
 
+  // Expensive, please use only in tests.
+  std::string ToString() const;
+  std::ostream& operator<<(std::ostream& os) const;
+
  private:
   uint32_t sum_;
   uint32_t bitrates_[kMaxSpatialLayers][kMaxTemporalStreams];