media/capture: Adding WebmMuxer class and unittests

media/filters/webm_muxer.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MEDIA_FILTERS_LIBWEBM_MUXER_H_
+#define MEDIA_FILTERS_LIBWEBM_MUXER_H_
+
+#include <set>
+
+#include "base/callback.h"
+#include "base/gtest_prod_util.h"
+#include "base/numerics/safe_math.h"
+#include "base/threading/thread_checker.h"
+#include "base/time/time.h"
+#include "media/base/media_export.h"
+#include "third_party/libwebm/source/mkvmuxer.hpp"
+#include "ui/gfx/geometry/size.h"
+
+namespace media {
+
+class MEDIA_EXPORT WebmMuxer : public mkvmuxer::IMkvWriter {
+ public:
+  typedef base::Callback<void(const char*, int)> WriteDataCB;

[miu, 2015/07/16 01:46:26]

Please document when and how this callback will be run.  I assume this is an
output byte array and a length?  Can the length be zero?  Does that indicate
EOF?

[mcasas, 2015/07/17 17:45:07]

Done. Clarified a bit the WebM restrictions in the class header;
most notably there's no container Trailer, so there's no EOF
indication "per-se".

+
+  explicit WebmMuxer(const WriteDataCB& write_data_callback);
+  ~WebmMuxer() override;
+
+  void OnEncodedVideo(int track_index,

[miu, 2015/07/16 01:46:25]

This method needs documentation.  Below, for AddVideoTrack(), you mention the
track index starts from one.  In other words, how does a client of this code
determine what |track_index| should be?

[mcasas, 2015/07/17 17:45:07]

Done.

+                      const std::string& encoded_data,
+                      const base::TimeTicks& timestamp,

[miu, 2015/07/16 01:46:26]

The caller should provide the media timestamp as a TimeDelta (where a zero
TimeDelta is the timestamp of the first frame).  This would be consistent with
other code in media/filters.  Then, this class wouldn't be responsible for any
time math, which could potentially cause flaky behavior since it has a
dependency on the system clock.

[mcasas, 2015/07/17 17:45:07]

Done. I like it.

+                      bool keyframe,
+                      const gfx::Size& frame_size,

[miu, 2015/07/16 01:46:26]

What happens when |frame_size| and/or |frame_rate| change between calls (for the
same video track)?

[mcasas, 2015/07/17 17:45:07]

As per offline discussion, what happens is that:
a) the first time a track_index is seen, |frame_size| and |frame_rate|
 are added as indications to it but
b) each individual encoded video frame header (SimpleBlock) has 
no indication of size and/or fps, being transcribed in the VPx
header itself.

Added to the method doco. Perhaps it would be more
interesting to publish CreateVideoTrack() and let the clients
specify those params ahead of OnEncodedVideo(), and 
change this last one's signature to

  void OnEncodedVideo(int track_number,
                      const std::string& encoded_data,
                      const base::TimeDelta& timedelta_us,
                      bool keyframe);

WDYT?

+                      double frame_rate);
+
+ private:
+  friend class WebmMuxerTest;
+  FRIEND_TEST_ALL_PREFIXES(WebmMuxerTest, Write);
+  FRIEND_TEST_ALL_PREFIXES(WebmMuxerTest, OnEncodedVideo);
+
+  // Creates and adds a new video track when receiving the first frame.
+  bool AddVideoTrack(const gfx::Size& frame_size,
+                     double frame_rate,
+                     int track_index_starting_from_one);
+
+  // IMkvWriter interface.
+  mkvmuxer::int32 Write(const void* buf, mkvmuxer::uint32 len) override;
+  mkvmuxer::int64 Position() const override;
+  mkvmuxer::int32 Position(mkvmuxer::int64 position) override;
+  bool Seekable() const override;
+  void ElementStartNotify(mkvmuxer::uint64 element_id,
+                          mkvmuxer::int64 position) override;
+
+  // Used to DCHECK that we are called on the correct thread.
+  base::ThreadChecker thread_checker_;
+
+  // Callback to dump written data as being called by libwebm.
+  const WriteDataCB write_data_callback_;
+
+  // Class creation time.
+  const base::TimeTicks time_start_;
+
+  // Rolling counter of the position in bytes of the written goo.
+  base::CheckedNumeric<uint64_t> position_;
+
+  // Local cache of track indexes to speed up adding a new track when seen for
+  // the first time.
+  std::set<int> track_indexes_;

[miu, 2015/07/16 01:46:26]

You don't need this since the same information can be accessed directly from
|segment_|.

[mcasas, 2015/07/17 17:45:07]

Done.

+
+  // Owned pointer to the MkvMuxer active element.
+  scoped_ptr<mkvmuxer::Segment> segment_;
+
+  DISALLOW_COPY_AND_ASSIGN(WebmMuxer);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_FILTERS_LIBWEBM_MUXER_H_