In the case of H264 we can't know which packet that is the fist packet of a
frame. In order to avoid creating incomplete frames we keep track of which
packets that we haven't received, and if there is a gap in the packet sequence
number leading up to this frame then a frame wont be created.
BUG=chromium:716558
Review-Url: https://codereview.webrtc.org/2926083002
Cr-Commit-Position: refs/heads/master@{#18559}