Replicate fast/events/mouse-event-buttons-attribute.html expectation to virtual/pointerevent/.
Replicate fast/events/mouse-event-buttons-attribute.html expectation to virtual/pointerevent/.

This test is also flaky in this virtual test suite, presumably for the same
reason.

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

Review URL: https://codereview.chromium.org/1126543006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@194892 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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