Deflake fast/css-generated-content/malformed-url.html

This test was flaky because it would sometimes print the url of the blocked
resource and sometimes not depending on when the navigation happened.

Instead we should wait for onload so that the url is always printed.

TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/221743006

git-svn-id: svn://svn.chromium.org/blink/trunk@170710 bbb929c8-8fbe-4397-9dbb-9b2b20218538