Preserve original document URL for XSSAuditor reports.

Otherwise, a history.replaceState() may erase the injected vector.

BUG=299132
R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/24816003

git-svn-id: svn://svn.chromium.org/blink/trunk@158406 bbb929c8-8fbe-4397-9dbb-9b2b20218538