Use one scoped_refptr.

Uses webrtc/base/scoped_ref_ptr.h and removes the copy in
system_wrappers.

BUG=
R=kwiberg@webrtc.org, tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1152733005

Cr-Commit-Position: refs/heads/master@{#9370}
25 files changed