Update jpeg writer to compile on iOS and document it better

webrtc/video/video_quality_test.cc

 /*
  *  Copyright (c) 2015 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.
  */
 #include "webrtc/video/video_quality_test.h"
@@ skipping 817 matching lines @@
     printf("RESULT cpu_usage: %s = %lf %%\n", test_label_.c_str(),
            GetCpuUsagePercent());
 
 #if defined(WEBRTC_WIN)
       // On Linux and Mac in Resident Set some unused pages may be counted.
       // Therefore this metric will depend on order in which tests are run and
       // will be flaky.
     PrintResult("memory_usage", memory_usage_, " bytes");
 #endif
 
-    // LibJpeg is not available on iOS.
-#if !defined(WEBRTC_IOS)
     // Saving only the worst frame for manual analysis. Intention here is to
     // only detect video corruptions and not to track picture quality. Thus,
     // jpeg is used here.
     if (FLAGS_save_worst_frame && worst_frame_) {
       std::string output_dir;
       test::GetTestOutputDir(&output_dir);
       std::string output_path =
           rtc::Pathname(output_dir, test_label_ + ".jpg").pathname();
       LOG(LS_INFO) << "Saving worst frame to " << output_path;
       test::JpegFrameWriter frame_writer(output_path);
       RTC_CHECK(frame_writer.WriteFrame(worst_frame_->frame,
                                         100 /*best quality*/));
     }
-#endif
 
     //  Disable quality check for quick test, as quality checks may fail
     //  because too few samples were collected.
     if (!is_quick_test_enabled_) {
       EXPECT_GT(psnr_.Mean(), avg_psnr_threshold_);
       EXPECT_GT(ssim_.Mean(), avg_ssim_threshold_);
     }
   }
 
   void PerformFrameComparison(const FrameComparison& comparison) {
@@ skipping 1295 matching lines @@
   if (!params_.logging.encoded_frame_base_path.empty()) {
     std::ostringstream str;
     str << receive_logs_++;
     std::string path =
         params_.logging.encoded_frame_base_path + "." + str.str() + ".recv.ivf";
     stream->EnableEncodedFrameRecording(rtc::CreatePlatformFile(path),
                                         100000000);
   }
 }
 }  // namespace webrtc