Refactor FFmpegVideoDecoder to try and generalize code common to all video decoders.

media/ffmpeg/ffmpeg_util.cc

+// Copyright (c) 2009 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.
+
+#include "media/ffmpeg/ffmpeg_util.h"
+
+#include "media/filters/ffmpeg_common.h"
+
+namespace media {
+
+static const AVRational kMicrosBase = { 1, base::Time::kMicrosecondsPerSecond };
+
+base::TimeDelta ConvertTimestamp(const AVRational& time_base, int64 timestamp) {
+  int64 microseconds = av_rescale_q(timestamp, time_base, kMicrosBase);
+  return base::TimeDelta::FromMicroseconds(microseconds);
+}
+
+}  // namespace media