Delay creation of decoders until they are needed

Before this CL, WebRTC created a decoder for each negotiated codec
profile. This quickly consumed all available HW decoder resources
on some platforms. This CL adds a field trial,
WebRTC-PreStreamDecoders, that makes it possible to set how many
decoders that should be created up front, from 0 to ALL. If the
field trial is set to 1, we only create a decoder for the
preferred codec. The other decoders are only created when they are
needed (i.e., if we receive the corresponding payload type).

Bug: webrtc:12462
Change-Id: I087571b540f6796d32d34923f9c7f8e89b0959c5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208284
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33300}

modules/video_coding/decoder_database.h

@@ -44,6 +44,7 @@ class VCMDecoderDataBase {
   bool DeregisterExternalDecoder(uint8_t payload_type);
   void RegisterExternalDecoder(VideoDecoder* external_decoder,
                                uint8_t payload_type);
+  bool IsExternalDecoderRegistered(uint8_t payload_type) const;
 
   bool RegisterReceiveCodec(uint8_t payload_type,
                             const VideoCodec* receive_codec,