Avoid log spam when decoder implementation changes

A refactoring (https://webrtc-review.googlesource.com/c/src/+/196520)
of decoder metadata handling introduced a bug which causes us to log an
info-level entry for every frame decoded if the implementation changes
during runtime (e.g. due to software fallback).

This CL fixes that to avoid spamming the logs.

Bug: webrtc:12271
Change-Id: I89016351b8752b259299c4cf56c6feddcca43460
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/211664
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33451}

modules/video_coding/generic_decoder.cc

@@ -272,7 +272,7 @@ int32_t VCMGenericDecoder::Decode(const VCMEncodedFrame& frame, Timestamp now) {
   if (decoder_info != decoder_info_) {
     RTC_LOG(LS_INFO) << "Changed decoder implementation to: "
                      << decoder_info.ToString();
-
+    decoder_info_ = decoder_info;
     _callback->OnDecoderImplementationName(
         decoder_info.implementation_name.empty()
             ? "unknown"