Revert to zero-initializing buffers for FFmpegVideoDecoder

      Following the regression on , I reverted back to PatchSet#5 from [1]
[0] https://code.google.com/p/chromium/issues/detail?id=514759
[1] https://codereview.chromium.org/1227383003

- Modified VideoFramePool::CreateFrame() to zero-initialize newly allocated frames.

BUG=514759

Review URL: https://codereview.chromium.org/1267003004

Cr-Commit-Position: refs/heads/master@{#343289}