Gardening. Try to deflake animation-matrix-negative-scale-unmatrix under virtual/threaded + debug.

As the start event now fires later and asynchronously it's possible to miss a
short animation entirely if there's enough jank.

NOTRY=true
TBR=timloh@chromium.org

Review URL: https://codereview.chromium.org/138103002

git-svn-id: svn://svn.chromium.org/blink/trunk@165060 bbb929c8-8fbe-4397-9dbb-9b2b20218538