Add unittest to check that ViECapturer does not hold on to frames after they have been delivered
Add unittest to check that ViECapturer does not hold on to frames after they have been delivered

This should have been part of the CL "Remove the need for scoped_ptr<I420VideoFrame> in VieCapturer" https://webrtc-codereview.appspot.com/43669004.

TBR=pbos,mflodman
BUG=1128

Review URL: https://webrtc-codereview.appspot.com/44629005

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@8680 4adac7df-926f-26a2-2b94-8c16560cd09d

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