Deflake smil-syncbase-self-dependency.svg

This patch deflakes the smil-syncbase-self-dependency.svg test that was
recently added as part of crbug.com/231531. This test has been flaky
since it was added [1].

[1] http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&showExpectations=true&tests=svg%2Fanimations%2Fsmil-syncbase-self-dependency.svg

BUG=231531
TBR=schenney

Review URL: https://codereview.chromium.org/145543005

git-svn-id: svn://svn.chromium.org/blink/trunk@165659 bbb929c8-8fbe-4397-9dbb-9b2b20218538