Add FullStack test for simulcast screenshare mode.

webrtc/video/vie_encoder.cc

Index: webrtc/video/vie_encoder.cc
diff --git a/webrtc/video/vie_encoder.cc b/webrtc/video/vie_encoder.cc
index c30752b6dfb048e96d40a5f2e2e25853fe4b529e..4591d89599a66f71d90adfaca33d6465d0645f89 100644
--- a/webrtc/video/vie_encoder.cc
+++ b/webrtc/video/vie_encoder.cc
@@ -438,12 +438,13 @@ void ViEEncoder::ReconfigureEncoder() {
                      static_cast<uint32_t>(max_data_payload_length_)) == VCM_OK;
   if (!success) {
     LOG(LS_ERROR) << "Failed to configure encoder.";
+    rate_allocator_.reset();
     RTC_DCHECK(success);
+  } else {
+    video_sender_.UpdateChannelParemeters(rate_allocator_.get(),
+                                          bitrate_observer_);
   }
 
-  video_sender_.UpdateChannelParemeters(rate_allocator_.get(),
-                                        bitrate_observer_);
-
   int framerate = stats_proxy_->GetSendFrameRate();
   if (framerate == 0)
     framerate = codec.maxFramerate;