Restore flaky expectation for virtual/pointerevent/fast/events/mouse-event-buttons-attribute.html.

See, e.g.
  http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win10/builds/770

This line was removed earlier here, but the test is still quite flaky:
  https://codereview.chromium.org/1259743005

BUG=469657
TBR=rbyers@chromium.org,ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1347293005

git-svn-id: svn://svn.chromium.org/blink/trunk@202633 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed