Reland of Work on flexible mode and screen sharing.

webrtc/video/video_send_stream.cc

Index: webrtc/video/video_send_stream.cc
diff --git a/webrtc/video/video_send_stream.cc b/webrtc/video/video_send_stream.cc
index 33e1f57575aa40fca810a7663b3b7b2710ff0b23..fd0906d657a362ce04af4af8710e661955ce1ae9 100644
--- a/webrtc/video/video_send_stream.cc
+++ b/webrtc/video/video_send_stream.cc
@@ -345,6 +345,12 @@
     if (config.encoder_specific_settings != nullptr) {
       video_codec.codecSpecific.VP9 = *reinterpret_cast<const VideoCodecVP9*>(
                                           config.encoder_specific_settings);
+      if (video_codec.mode == kScreensharing) {
+        video_codec.codecSpecific.VP9.flexibleMode = true;
+        // For now VP9 screensharing use 1 temporal and 2 spatial layers.
+        RTC_DCHECK_EQ(video_codec.codecSpecific.VP9.numberOfTemporalLayers, 1);
+        RTC_DCHECK_EQ(video_codec.codecSpecific.VP9.numberOfSpatialLayers, 2);
+      }
     }
     video_codec.codecSpecific.VP9.numberOfTemporalLayers =
         static_cast<unsigned char>(