Reland "VP9 screenshare: Don't base layers frame-rate on input frame-rate"

Reland with fixes.

If input framerate is a little unstable, using it to cap layers will
make output framerate even smaller for longer periods of time.

Also, fix screenshare_loopback test for low-fps vp9 testing.

Bug: webrtc:10257
Change-Id: Id40a780d461e6b51cb44d275b8aa5d7b348d3586
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/138215
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28054}
diff --git a/video/video_quality_test.cc b/video/video_quality_test.cc
index 5415f96..cc30c79 100644
--- a/video/video_quality_test.cc
+++ b/video/video_quality_test.cc
@@ -843,7 +843,7 @@
             params_.ss[video_idx].num_spatial_layers);
         vp9_settings.interLayerPred = params_.ss[video_idx].inter_layer_pred;
         // High FPS vp9 screenshare requires flexible mode.
-        if (params_.video[video_idx].fps > 5) {
+        if (params_.ss[video_idx].num_spatial_layers > 1) {
           vp9_settings.flexibleMode = true;
         }
         video_encoder_configs_[video_idx].encoder_specific_settings =