Cast: Missing field in IPC message

The IPC message for sending encoded video frames is missing the field
for RTP timestamp. Receiver does not play without this information.

Test: Runing cast.streaming APIs against cast_receiver_app and it should
render the video stream.

R=hubbe@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/178223005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253621 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253621