Assign issue number to css-contain/contain-layout-button-001.html

It looks like dholbert thinks the test is wrong, see the
description of crbug.com/965740.

TBR=cbiesinger@chromium.org
NOTRY=true

Bug: 965740
Change-Id: I60ffe13c36962fb97e0acf0ad37ca097ba6955e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634513
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664176}