Prepare for deleting VideoCodec::plType

Deletes all webrtc usage of this member. Next step is to delete
any downstream references, and when that's done, the member can be
deleted.

Bug: None
Change-Id: I3f3a94a063dccf56468a1069653efd3809875b01
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/181201
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31911}

modules/video_coding/decoder_database.h

@@ -76,6 +76,7 @@ class VCMDecoderDataBase {
   const VCMExtDecoderMapItem* FindExternalDecoderItem(
       uint8_t payload_type) const;
 
+  uint8_t current_payload_type_;  // Corresponding to receive_codec_.
   VideoCodec receive_codec_;
   std::unique_ptr<VCMGenericDecoder> ptr_decoder_;
   DecoderMap dec_map_;