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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=154190

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

git-svn-id: svn://svn.chromium.org/blink/trunk@154193 bbb929c8-8fbe-4397-9dbb-9b2b20218538