Fix the default value of offerToReceiveAudio and offerToReceiveVideo.
According to the most recent discussion on https://github.com/rtcweb-wg/jsep/issues/66, JS should not be allowed to set negative value and the 'unset' state needs to be passed down as a special value '-1'.

BUG=https://code.google.com/p/webrtc/issues/detail?id=3282

Review URL: https://codereview.chromium.org/429893008

git-svn-id: svn://svn.chromium.org/blink/trunk@179276 bbb929c8-8fbe-4397-9dbb-9b2b20218538