Cast: Split kDuplicatePacketReceived into separate events for audio/video.
This change brings this type of event in line with the rest - having separate types
for audio / video. As a new type that is potentially sent over in RTCP is introduced,
a new RTCP encoding for that type is added as well.
This change is backwards-compatible because these events aren't being sent over in RTCP because they are not processed by ReceiverRtcpEventSubscriber.
BUG=337901
Review URL: https://codereview.chromium.org/
164253003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251301 0039d316-1c4b-4281-b951-d872f2087c98