libvpx

[twopass temporal svc] Fix decoding error on seek.

Don't put small empty frame in front of a key frame. We will
put key frame flag in webm container if there's a visible key
frame. But there will be decoding error when we seek to here
if we put the small empty frame, which will be inter frame,
in front of it.

Change-Id: Id50c2c1fd31da0405ff6faa7375cc2f49c55402d