This makes "WebRTC-AllowDisablingLegacyScalability" enabled-by-default,
meaning any app can opt-in to spec-compliant simulcast when
scalabilityMode is specified.
The opt-in criteria is also made more restricitve: you now have to
specify both scalabilityMode and scaleResolutionDownBy to get simulcast,
otherwise you continue to get legacy "single stream" path.
The reason for this is not to cause any surprises in use cases like
[{scalabilityMode:"L1T1", active:true}, {active:false}, {active:false}]
In cases like this where scaleResolutionDownBy is not specified, it
defaults to 4:2:1 if simulcast is used but the legacy path caps it to
one stream, meaning full resolution. By restricing simulcast only to
cases that set scaleResolutionDownBy, we remove the risk of an app
getting a different resolution than expected due to opt-in.
Bug: webrtc:14884, webrtc:15005
Change-Id: I5efb87af60afaeb1e3ff76698d887aaa1f9d63a9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/298922
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39660}
adb946054c