Initialize memory in I420VideoFrame unittest

Previously, when CreateEmptyFrame was called with a smaller size than before, we would reuse the allocation. Now, we allocate a new tight frame. The CL that made this change is https://webrtc-codereview.appspot.com/42469004/. This exposed an uninitialized memory problem in a I420VideoFrame unittest. This CL fixes that unittest.

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/42619004

Cr-Commit-Position: refs/heads/master@{#8593}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8593 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed