By default, we'll now offer to receive if already receiving (meaning that the last remote description contained a track). Also, m-lines that are neither receiving nor sending are now correctly marked "inactive". Also moved some logic relating to default tracks out of webrtcsdp.cc, such that now the direction seen by upper layers will always be consistent with the consumed/produced SDP. BUG=528089 Review URL: https://codereview.webrtc.org/1406803004 Cr-Commit-Position: refs/heads/master@{#10376}