MSE: Update FrameProcessor comments w.r.t. current spec

The spec's coded frame processing algorithm steps have changed slightly.
This change updates the related code comments to link to the June 9,
2016 editor's draft, adds a TODO referencing bug 398141, and removes a
TODO referencing bug 351166.

BUG=398141,351166
R=chcunningham@chromium.org

Review-Url: https://codereview.chromium.org/2064073004
Cr-Commit-Position: refs/heads/master@{#399969}
1 file changed