Add experimental simulcast screen content mode

BUG=webrtc:4172

Review-Url: https://codereview.webrtc.org/2636443002
Cr-Commit-Position: refs/heads/master@{#16135}
diff --git a/webrtc/modules/video_coding/video_codec_initializer.cc b/webrtc/modules/video_coding/video_codec_initializer.cc
index dbdede0..9d15952 100644
--- a/webrtc/modules/video_coding/video_codec_initializer.cc
+++ b/webrtc/modules/video_coding/video_codec_initializer.cc
@@ -38,8 +38,8 @@
     case kVideoCodecVP8: {
       if (!codec->VP8()->tl_factory) {
         if (codec->mode == kScreensharing &&
-            codec->numberOfSimulcastStreams == 1 &&
-            codec->VP8()->numberOfTemporalLayers == 2) {
+            codec->numberOfSimulcastStreams >= 1 &&
+            codec->VP8()->numberOfTemporalLayers >= 1) {
           // Conference mode temporal layering for screen content.
           tl_factory.reset(new ScreenshareTemporalLayersFactory());
         } else {