Initialize/configure video encoders asychronously.

webrtc/video/video_send_stream.h

Index: webrtc/video/video_send_stream.h
diff --git a/webrtc/video/video_send_stream.h b/webrtc/video/video_send_stream.h
index 88d8abc4fb647d7c49940bbc48e627b1ef6a6592..dd11d16b1a99a4db77026cbfee8039a9a382d760 100644
--- a/webrtc/video/video_send_stream.h
+++ b/webrtc/video/video_send_stream.h
@@ -14,6 +14,7 @@
 #include <map>
 #include <vector>
 
+#include "webrtc/base/criticalsection.h"
 #include "webrtc/call.h"
 #include "webrtc/common_video/libyuv/include/webrtc_libyuv.h"
 #include "webrtc/video/encoded_frame_callback_adapter.h"
@@ -75,6 +76,10 @@ class VideoSendStream : public webrtc::VideoSendStream,
   int GetPaddingNeededBps() const;
 
  private:
+  struct EncoderSettings {
+    VideoCodec video_codec;
+    int min_transmit_bitrate_bps;
+  };
   static bool EncoderThreadFunction(void* obj);
   void EncoderProcess();
 
@@ -93,6 +98,9 @@ class VideoSendStream : public webrtc::VideoSendStream,
   rtc::PlatformThread encoder_thread_;
   rtc::Event encoder_wakeup_event_;
   volatile int stop_encoder_thread_;
+  rtc::CriticalSection encoder_settings_crit_;
+  rtc::Optional<EncoderSettings> pending_encoder_settings_
+      GUARDED_BY(encoder_settings_crit_);
 
   OveruseFrameDetector overuse_detector_;
   PayloadRouter payload_router_;