Fix potential incorrect sync flags used with screenshare TL stream.

webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc

Index: webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
diff --git a/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc b/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
index 8a33205d30b5c74af0ef8e837179935f7767311b..397e005cbe0f12442b20caa69ceda8fb60b33f46 100644
--- a/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
+++ b/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
@@ -152,11 +152,17 @@ TemporalLayers::FrameConfig ScreenshareLayers::UpdateLayerConfig(
       last_emitted_tl0_timestamp_ = unwrapped_timestamp;
       break;
     case 1:
-      if (TimeToSync(unwrapped_timestamp)) {
-        last_sync_timestamp_ = unwrapped_timestamp;
-        layer_state = TemporalLayerState::kTl1Sync;
+      if (layers_[1].state != TemporalLayer::State::kDropped) {
+        if (TimeToSync(unwrapped_timestamp)) {
+          last_sync_timestamp_ = unwrapped_timestamp;
+          layer_state = TemporalLayerState::kTl1Sync;
+        } else {
+          layer_state = TemporalLayerState::kTl1;
+        }
       } else {
-        layer_state = TemporalLayerState::kTl1;
+        layer_state = last_sync_timestamp_ == unwrapped_timestamp
+                          ? TemporalLayerState::kTl1Sync
+                          : TemporalLayerState::kTl1;
       }
       break;
     case -1: