Unflake sandboxed-iframe-autofocus-denied.html.

The previous version of this test generated non-deterministic results
due to the way console messages are generated. Specifically, the test
case was constructed while parsing might not yet be complete, but the
act of constructing the test case generated console messages. If a
console message is generated with no specified line number,
Document::internalAddMessage attempts to attach a line number, but one
of the conditionals for doing so is whether the Document is still
parsing making the output non-deterministic.

To get around that, the test  has been rewritten to only run once the
document is finished loading.

BUG=381451
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/314393002

git-svn-id: svn://svn.chromium.org/blink/trunk@175706 bbb929c8-8fbe-4397-9dbb-9b2b20218538