Update jpeg writer to compile on iOS and document it better

webrtc/test/testsupport/jpeg_frame_writer.cc

 /*
  *  Copyright (c) 2017 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 17 matching lines @@
 
 namespace webrtc {
 namespace test {
 
 JpegFrameWriter::JpegFrameWriter(const std::string &output_filename)
     : frame_written_(false),
       output_filename_(output_filename),
       output_file_(nullptr) {}
 
 bool JpegFrameWriter::WriteFrame(const VideoFrame& input_frame, int quality) {
-  RTC_CHECK(!frame_written_) << "Only a single frame can be saved to Jpeg.";
+  if (frame_written_) {
+    LOG(LS_ERROR) << "Only a single frame can be saved to Jpeg.";
+    return false;
+  }
   const int kColorPlanes = 3;  // R, G and B.
   size_t rgb_len = input_frame.height() * input_frame.width() * kColorPlanes;
   std::unique_ptr<uint8_t[]> rgb_buf(new uint8_t[rgb_len]);
 
   // kRGB24 actually corresponds to FourCC 24BG which is 24-bit BGR.
   if (ConvertFromI420(input_frame, VideoType::kRGB24, 0, rgb_buf.get()) < 0) {
     LOG(LS_ERROR) << "Could not convert input frame to RGB.";
     return false;
   }
   output_file_ = fopen(output_filename_.c_str(), "wb");
@@ skipping 29 matching lines @@
   jpeg_finish_compress(&cinfo);
   jpeg_destroy_compress(&cinfo);
   fclose(output_file_);
 
   frame_written_ = true;
   return true;
 }
 
 }  // namespace test
 }  // namespace webrtc