[PPAPI] Implement media stream video track API

ppapi/shared_impl/media_stream_frame.h

+// Copyright (c) 2014 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 PPAPI_SHARED_IMPL_MEDIA_STREAM_FRAME_H_
+#define PPAPI_SHARED_IMPL_MEDIA_STREAM_FRAME_H_
+
+#include "ppapi/c/ppb_video_frame.h"
+
+namespace ppapi {
+
+union MediaStreamFrame {
+  enum Type {
+    TYPE_UNKNOWN = 0,
+    TYPE_AUDIO = 1,
+    TYPE_VIDEO = 2,
+  };
+
+  struct Header {
+    Type type;
+    uint32_t size;
+  };
+  PP_COMPILE_ASSERT_SIZE_IN_BYTES(Header, 8);

[dmichael (off chromium), 2014/01/14 16:12:03]

It would be helpful to readers to comment on why it's important to have
consistent size... E.g.:
"""
Because these structs are written and read in shared memory, we need the size
and alighment to be consistent between NaCl and its host trusted platform.
"""

[Peng, 2014/01/14 21:31:35]

Done.

+
+  struct Audio {
+    Header header;
+    // TODO(penghuang): implement the audio frame.
+  };
+  PP_COMPILE_ASSERT_SIZE_IN_BYTES(Audio, 8);
+
+  struct Video {
+    Header header;
+    PP_TimeDelta timestamp;
+    PP_VideoFrame_Format format;
+    PP_Size size;
+    uint32_t data_size;
+    // Uses 8 bytes to make sure below compile assert work in all platforms.

[dmichael (off chromium), 2014/01/14 16:12:03]

I would maybe rephrase to: "Uses 8 bytes to make sure the Video struct has
consistent size between NaCl code and renderer code."

[Peng, 2014/01/14 21:31:35]

Done.

+    uint8_t data[8];
+  };
+  PP_COMPILE_ASSERT_SIZE_IN_BYTES(Video, 40);
+
+  Header header;
+  Video video;
+  Audio audio;
+};
+
+}  // namespace ppapi
+
+#endif  // PPAPI_SHARED_IMPL_MEDIA_STREAM_FRAME_H_