Update jpeg writer to compile on iOS and document it better

webrtc/video/replay.cc

 /*
  *  Copyright (c) 2014 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 160 matching lines @@
     if (renderer_)
       renderer_->OnFrame(video_frame);
 
     if (basename_.empty())
       return;
 
     std::stringstream filename;
     filename << basename_ << count_++ << "_" << video_frame.timestamp()
              << ".jpg";
 
-#if !defined(WEBRTC_IOS)
     test::JpegFrameWriter frame_writer(filename.str());
     RTC_CHECK(frame_writer.WriteFrame(video_frame, 100));
-#endif
   }
 
   const std::string basename_;
   rtc::VideoSinkInterface<VideoFrame>* const renderer_;
   FILE* file_;
   size_t count_;
 };
 
 class DecoderBitstreamFileWriter : public EncodedFrameObserver {
  public:
@@ skipping 135 matching lines @@
 }
 }  // namespace webrtc
 
 int main(int argc, char* argv[]) {
   ::testing::InitGoogleTest(&argc, argv);
   google::ParseCommandLineFlags(&argc, &argv, true);
 
   webrtc::test::RunTest(webrtc::RtpReplay);
   return 0;
 }