If more than 60 frames are created and not returned, the implementation will crash.
I420BufferPool are currently used by the VP8 decoder, Quality scaler and VideoFrameFactory.
BUG=b/31390397
NOTRY=true // Because of failing gclient runhooks on some bots
Review-Url: https://codereview.webrtc.org/2370653003
Cr-Commit-Position: refs/heads/master@{#14395}