Make sure we process all pending offer/answer requests before terminating.
This fixes a bug in the WebRtcSessionDescriptionFactory where messages would be dropped or worse yet processed after the factory was deleted.

BUG=chromium:507307

Review URL: https://codereview.webrtc.org/1231823002

Cr-Commit-Position: refs/heads/master@{#9557}
7 files changed