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Previously PepperPortAllocator was using only the first STUN server
because cricket::PortConfiguration didn't support more than one STUN
server.

Review URL: https://codereview.chromium.org/456013002

Cr-Commit-Position: refs/heads/master@{#289143}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289143 0039d316-1c4b-4281-b951-d872f2087c98