Update jpeg writer to compile on iOS and document it better

webrtc/test/testsupport/jpeg_frame_writer_ios.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.
+ */
+
+#include "webrtc/rtc_base/checks.h"
+#include "webrtc/rtc_base/logging.h"
+#include "webrtc/test/testsupport/frame_writer.h"
+
+
+namespace webrtc {
+namespace test {
+
+JpegFrameWriter::JpegFrameWriter(const std::string& /*output_filename*/) {}
+
+bool JpegFrameWriter::WriteFrame(const VideoFrame& /*input_frame*/,
+                                 int /*quality*/) {
+  LOG(LS_WARNING) << "Libjpeg isn't available on IOS. Jpeg frame writer is not "
+      "supported. No frame will be saved.";
+  // Don't fail.
+  return true;
+}
+
+}  // namespace test
+}  // namespace webrtc