Don't send accessibility notifications for a doc with an empty/blank url.

This is a refinement to the change that started sending
accessibility notifications early. This also addresses
the test for this change that was flaky.

BUG=353067,363235
NOTRY=true

Review URL: https://codereview.chromium.org/242103012

git-svn-id: svn://svn.chromium.org/blink/trunk@172948 bbb929c8-8fbe-4397-9dbb-9b2b20218538