Oilpan: Move RTCOfferOptions to oilpan's heap

This is one of leftovers when we shipped Oilpan for mediastream/.

BUG=340522

Review URL: https://codereview.chromium.org/510673003

git-svn-id: svn://svn.chromium.org/blink/trunk@181162 bbb929c8-8fbe-4397-9dbb-9b2b20218538