Running video loopback on https://appr.tc/ revealed that it is possible
to use the same SSRC for a local and remote audio or video track. This
caused a DCHECK crash. The constructor of TrackMediaInfoMap is updated
to support this mapping and the unittest is updated (moved and modified
a test from being a death test to being a non-death test).
I've verified that this fixes the bug.
BUG=chromium:693087
Review-Url: https://codereview.webrtc.org/2703783002
Cr-Commit-Position: refs/heads/master@{#16713}