Add stereo codec header and pass it through RTP

- Defines CodecSpecificInfoStereo that carries stereo specific header info from
encoded image.
- Defines RTPVideoHeaderStereo that carries the above info to packetizer,
see module_common_types.h.
- Adds an RTPPacketizer and RTPDepacketizer that supports passing specific stereo
header.
- Uses new data containers in StereoAdapter classes.

This CL is the step 3 for adding alpha channel support over the wire in webrtc.
See https://webrtc-review.googlesource.com/c/src/+/7800 for the experimental
CL that gives an idea about how it will come together.
Design Doc: https://goo.gl/sFeSUT

Bug: webrtc:7671
Change-Id: Ia932568fdd7065ba104afd2bc0ecf25a765748ab
Reviewed-on: https://webrtc-review.googlesource.com/22900
Reviewed-by: Emircan Uysaler <emircan@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20920}

modules/video_coding/codecs/test/video_codec_test.h

@@ -12,6 +12,7 @@
 #define MODULES_VIDEO_CODING_CODECS_TEST_VIDEO_CODEC_TEST_H_
 
 #include <memory>
+#include <vector>
 
 #include "api/video_codecs/video_decoder.h"
 #include "api/video_codecs/video_encoder.h"
@@ -32,6 +33,7 @@ class VideoCodecTest : public ::testing::Test {
         decode_complete_callback_(this),
         encoded_frame_event_(false /* manual reset */,
                              false /* initially signaled */),
+        wait_for_encoded_frames_threshold_(1),
         decoded_frame_event_(false /* manual reset */,
                              false /* initially signaled */) {}
 
@@ -74,8 +76,19 @@ class VideoCodecTest : public ::testing::Test {
 
   void SetUp() override;
 
+  // Helper method for waiting a single encoded frame.
   bool WaitForEncodedFrame(EncodedImage* frame,
                            CodecSpecificInfo* codec_specific_info);
+
+  // Helper methods for waiting for multiple encoded frames. Caller must
+  // define how many frames are to be waited for via |num_frames| before calling
+  // Encode(). Then, they can expect to retrive them via WaitForEncodedFrames().
+  void SetWaitForEncodedFramesThreshold(size_t num_frames);
+  bool WaitForEncodedFrames(
+      std::vector<EncodedImage>* frames,
+      std::vector<CodecSpecificInfo>* codec_specific_info);
+
+  // Helper method for waiting a single decoded frame.
   bool WaitForDecodedFrame(std::unique_ptr<VideoFrame>* frame,
                            rtc::Optional<uint8_t>* qp);
 
@@ -95,9 +108,11 @@ class VideoCodecTest : public ::testing::Test {
 
   rtc::Event encoded_frame_event_;
   rtc::CriticalSection encoded_frame_section_;
-  rtc::Optional<EncodedImage> encoded_frame_
+  size_t wait_for_encoded_frames_threshold_;
+  std::vector<EncodedImage> encoded_frames_
+      RTC_GUARDED_BY(encoded_frame_section_);
+  std::vector<CodecSpecificInfo> codec_specific_infos_
       RTC_GUARDED_BY(encoded_frame_section_);
-  CodecSpecificInfo codec_specific_info_ RTC_GUARDED_BY(encoded_frame_section_);
 
   rtc::Event decoded_frame_event_;
   rtc::CriticalSection decoded_frame_section_;