Use data:, rather than about:blank as a substitute form action so the resulting blank page will have an unique origin.

This is similar to the work we did in XSSAuditorDelegate for the mode=block
case, where we used the SecurityOrigin::urlWithUniqueOrign constant.  We can't
use that here due to threading.

Testing is covered by rebasing the existing test cases.
BUG=331060
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/124973004

git-svn-id: svn://svn.chromium.org/blink/trunk@164538 bbb929c8-8fbe-4397-9dbb-9b2b20218538