Drop DocumentCreateEventVRDisplayEvent use counter

These entries are no longer needed. Context:
https://codereview.chromium.org/2265053004
https://codereview.chromium.org/2210253002

Also sneek in the M54 cutoff comment.

BUG=389343,569690
R=bajones@chromium.org

Review-Url: https://codereview.chromium.org/2301533002
Cr-Commit-Position: refs/heads/master@{#416122}
1 file changed