To differentiate between when a packet should be dropped or when a keyframe
is missing its SPS and/or SPS CopyAndFixBitstream now returns:
- kInsert, the packet should be inserted into the PacketBuffer.
- kDrop, the packet should be dropped.
- kRequestKeyframe, a keyframe should be requested.
BUG=webrtc:5514
Review-Url: https://codereview.webrtc.org/2477343002
Cr-Commit-Position: refs/heads/master@{#14948}