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This makes it possible to add candidate configurations at either the
back (least-preferred) or front (most-preferred) of each channel's
candidate list.

This CL also disables VP9, by default, in candidate session
configurations, replaces DisableVideoCodec with 
EnableVideoCodec, and makes the Enable/Disable<foo>Codec
helpers members of CandidateSessionConfig..

Review URL: https://codereview.chromium.org/292093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272324 0039d316-1c4b-4281-b951-d872f2087c98