Reland "Enable quality scaling when allowed"

This reverts commit 609b524dd3ff36719b5c4470b85d37dcdadfb1f8.

Reason for revert: Disable QualityScalingAllowed_QualityScalingEnabled on iOS.

Original change's description:
Before this CL quality scaling was conditioned on scaling settings
provided by encoder. That should not be a requirement since encoder
may not be aware of quality scaling which is a WebRTC feature. In M90
chromium HW encoders do not provide scaling settings (chromium:1179020).
The default scaling settings provided by these encoders are not correct
(b/181537172).

This CL adds is_quality_scaling_allowed to VideoEncoderConfig. The flag
is set to true in singlecast with normal video feed (not screen sharing)
mode. If quality scaling is allowed it is enabled no matter whether
scaling settings are present in encoder info or not. Setting from
QualityScalingExperiment are used in case if not provided by encoder.

Bug: chromium:1179020
Bug: webrtc:12511
Change-Id: Ia0923e5a62acdfdeb06f9aad5d670be8a0f8d746
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/209643
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33385}

api/video_codecs/video_encoder_config.cc

@@ -57,7 +57,8 @@ VideoEncoderConfig::VideoEncoderConfig()
       max_bitrate_bps(0),
       bitrate_priority(1.0),
       number_of_streams(0),
-      legacy_conference_mode(false) {}
+      legacy_conference_mode(false),
+      is_quality_scaling_allowed(false) {}
 
 VideoEncoderConfig::VideoEncoderConfig(VideoEncoderConfig&&) = default;