Initialize memory in I420VideoFrame unittest
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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8593}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f82109cb4d638826f8c795a9bcf41a51c258ba2d

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