Un-breaks peerconnection_client which would instantly use-after-free on
an allocated VCM because it wasn't building a scoped_refptr so all
references to the VCM were dropped.
BUG=webrtc:5229
TBR=tommi@webrtc.org
TEST=Run peerconnection_client locally, verify that there's no crash.
Review URL: https://codereview.webrtc.org/1817953005 .
Cr-Commit-Position: refs/heads/master@{#12088}