Add FullStack test for simulcast screenshare mode.

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

 /*
  *  Copyright (c) 2016 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.
  */
 
@@ skipping 12 matching lines @@
     const VideoCodec& codec,
     std::unique_ptr<TemporalLayersFactory> tl_factory)
     : codec_(codec), tl_factory_(std::move(tl_factory)) {
   if (tl_factory_.get())
     tl_factory_->SetListener(this);
 }
 
 void SimulcastRateAllocator::OnTemporalLayersCreated(int simulcast_id,
                                                      TemporalLayers* layers) {
   RTC_DCHECK(temporal_layers_.find(simulcast_id) == temporal_layers_.end());
+  RTC_DCHECK(layers);
   temporal_layers_[simulcast_id] = layers;
 }
 
 BitrateAllocation SimulcastRateAllocator::GetAllocation(
     uint32_t total_bitrate_bps,
     uint32_t framerate) {
   uint32_t left_to_allocate = total_bitrate_bps;
   if (codec_.maxBitrate && codec_.maxBitrate * 1000 < left_to_allocate)
     left_to_allocate = codec_.maxBitrate * 1000;
 
@@ skipping 110 matching lines @@
   BitrateAllocation allocation =
       temp_allocator.GetAllocation(codec_.maxBitrate * 1000, framerate);
   return allocation.get_sum_bps();
 }
 
 const VideoCodec& webrtc::SimulcastRateAllocator::GetCodec() const {
   return codec_;
 }
 
 }  // namespace webrtc