A previous change, https://gerrit.chromium.org/gerrit/#/c/70632,
introduced a size validation for reference frames to insuare the
input stream is a valid VP9 stream. However, the logic requiring
all reference frames have valid size turned out to be too strict.

In this commit, we modify the validation to require one of the
reference frame has valid dimension. In addition, the decoder
reports error whenever it detects the use of reference frame
with invalid scalig ratio.

Change-Id: If8efc312244087556cfe00f1fcbdff811268ebad