Remove the need for scoped_ptr<I420VideoFrame> in VieCapturer.
This adds the method I420VideoFrame::Reset and replace the use of scoped_ptr in ViECapturer.
Also, a unittest is added to check that ViECapturer does not retain a frame after it has been delivered.
BUG=1128
R=mflodman@webrtc.org, pbos@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/43669004
Cr-Commit-Position: refs/heads/master@{#8678}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8678 4adac7df-926f-26a2-2b94-8c16560cd09d