svg/custom/getsvgdocument.html is flakey

See the flakiness dashboard:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=svg%2Fcustom%2Fgetsvgdocument.html

The cause is explained in crbug.com/284734 .

BUG=284734

Review URL: https://chromiumcodereview.appspot.com/23598009

git-svn-id: svn://svn.chromium.org/blink/trunk@157404 bbb929c8-8fbe-4397-9dbb-9b2b20218538