[Cast] Correct serialization of packet sequence number.

Code that modifies an already-serialized packet was erroneously writing
out a 16-bit value using little-endian byte order.  This change corrects
that to be written big-endian, per the spec.

BUG=385894

Review URL: https://codereview.chromium.org/342003007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278335 0039d316-1c4b-4281-b951-d872f2087c98

media/cast/transport/rtp_sender/rtp_sender.cc

@@ -4,6 +4,7 @@
 
 #include "media/cast/transport/rtp_sender/rtp_sender.h"
 
+#include "base/big_endian.h"
 #include "base/logging.h"
 #include "base/rand_util.h"
 #include "media/cast/transport/cast_transport_defines.h"
@@ -134,10 +135,13 @@ void RtpSender::ResendPackets(
 }
 
 void RtpSender::UpdateSequenceNumber(Packet* packet) {
-  uint16 new_sequence_number = packetizer_->NextSequenceNumber();
-  int index = 2;
-  (*packet)[index] = (static_cast<uint8>(new_sequence_number));
-  (*packet)[index + 1] = (static_cast<uint8>(new_sequence_number >> 8));
+  // TODO(miu): This is an abstraction violation.  This needs to be a part of
+  // the overall packet (de)serialization consolidation.
+  static const int kByteOffsetToSequenceNumber = 2;
+  base::BigEndianWriter big_endian_writer(
+      reinterpret_cast<char*>((&packet->front()) + kByteOffsetToSequenceNumber),
+      sizeof(uint16));
+  big_endian_writer.WriteU16(packetizer_->NextSequenceNumber());
 }
 
 }  // namespace transport