Fix potential incorrect sync flags used with screenshare TL stream.

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

 /* Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */
 
 #include "webrtc/modules/video_coding/codecs/vp8/screenshare_layers.h"
@@ skipping 134 matching lines @@
       active_layer_ = 0;
     }
   }
 
   switch (active_layer_) {
     case 0:
       layer_state = TemporalLayerState::kTl0;
       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:
       layer_state = TemporalLayerState::kDrop;
       ++stats_.num_dropped_frames_;
       break;
     default:
       RTC_NOTREACHED();
   }
 
@@ skipping 290 matching lines @@
       RTC_HISTOGRAM_COUNTS_10000("WebRTC.Video.Screenshare.Layer1.Qp",
                                  stats_.tl1_qp_sum_ / stats_.num_tl1_frames_);
       RTC_HISTOGRAM_COUNTS_10000(
           "WebRTC.Video.Screenshare.Layer1.TargetBitrate",
           stats_.tl1_target_bitrate_sum_ / stats_.num_tl1_frames_);
     }
   }
 }
 
 }  // namespace webrtc