Update jpeg writer to compile on iOS and document it better

webrtc/test/testsupport/frame_writer.h

 /*
  *  Copyright (c) 2012 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 65 matching lines @@
                      int height,
                      int frame_rate);
   ~Y4mFrameWriterImpl() override;
   bool Init() override;
   bool WriteFrame(uint8_t* frame_buffer) override;
 
  private:
   const int frame_rate_;
 };
 
-// LibJpeg is not available on iOS
-#if !defined(is_ios)
+// LibJpeg is not available on iOS. This class will do nothing on iOS.
 class JpegFrameWriter {
  public:
   JpegFrameWriter(const std::string &output_filename);
+  // Quality can be from 0 (worst) to 100 (best). Best quality is still lossy.
+  // WriteFrame can be called only once. Subsequent calls will fail.
   bool WriteFrame(const VideoFrame& input_frame, int quality);
 
+#if !defined(WEBRTC_IOS)
  private:
   bool frame_written_;
   const std::string output_filename_;
   FILE* output_file_;
+#endif
 };
-#endif
 
 }  // namespace test
 }  // namespace webrtc
 
 #endif  // WEBRTC_TEST_TESTSUPPORT_FRAME_WRITER_H_