Reland "Only enable conference mode simulcast allocations with flag enabled"

This is a reland of 32ca95145c4636374266f5b5d4d1ac43658bc758

Fix includes not enabling the screenshare conference behavior on non
screenshare sources even if the flag is enabled.

Original change's description:
> Only enable conference mode simulcast allocations with flag enabled
>
> Non-conference mode simulcast screenshares were mistakenly using the
> conference mode semantics in the simulcast rate allocator, which broke
> spec compliant usage in some situation.
>
> This behavior should only be used when explicitly using the SDP entry
> "a=x-google-flag:conference" in both offer and answer.
>
> Bug: webrtc:11310, chromium:1093819
> Change-Id: Ibcba75c88a8405d60467546b33977a782e04e469
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/179081
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Commit-Queue: Florent Castelli <orphis@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31828}

Bug: webrtc:11310
Bug: chromium:1093819
Change-Id: Ic933f93a5c4bad20583354fe821f8a1170e911cd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/180802
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31847}

modules/video_coding/video_codec_initializer.cc

@@ -68,6 +68,10 @@ VideoCodec VideoCodecInitializer::VideoEncoderConfigToVideoCodec(
       break;
   }
 
+  video_codec.legacy_conference_mode =
+      config.content_type == VideoEncoderConfig::ContentType::kScreen &&
+      config.legacy_conference_mode;
+
   // TODO(nisse): The plType field should be deleted. Luckily, our
   // callers don't need it.
   video_codec.plType = 0;