This is an experiment to see if it clears up bot flakiness. Add an onload handler for the <use> element in this test, to make sure the resource load has finished before content_shell takes the image snapshot.

TBR=ojan
BUG=258991

Review URL: https://codereview.chromium.org/18942004

git-svn-id: svn://svn.chromium.org/blink/trunk@154190 bbb929c8-8fbe-4397-9dbb-9b2b20218538