Make the BlockDifference() functions return DiffInfo as their callers expect.

This prevents the compiler from warning about possible type truncations at the
callsites.

BUG=chromium:81439
TEST=none
R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/41059004

Cr-Commit-Position: refs/heads/master@{#9211}
7 files changed