This will create another decoder instance, which isn't ideal, but
that's better than the current behavior where things don't work at all.
We still need to fix the root cause of the linked bug, which is that we
don't remember previous payload type mappings when generating an offer.
This CL also adds a check for what is a real error: when a payload type
that was mapped to one codec is changed to map to a different codec.
And lastly, this CL removes a DCHECK for an assumption that was not
valid: that subsequently applied codec lists will always be supersets of
previous lists.
BUG=webrtc:5847
Review-Url: https://codereview.webrtc.org/2831333002
Cr-Commit-Position: refs/heads/master@{#17897}
cb3836773c