Implement NavigationScheduler::schedulePageBlock() as a redirect to empty substitute data.

This replaces the long-standing kludge of navigating to "data:," so that
we preserve the URL of the page that was blocked. Otherwise, cross-origin
detection of the XSSAuditor is possible via a variety of techniques owing
to the change in the URL.

We lose the benefit of the unique origin, however. I don't think actually
provides any benefit, if only blank content is going into the replacement
page. As a consequence, the parent frame will successfully see same-origin
content in some of the tests. The cross-origin test remains unmodified, 
showing that there aren't new leaks (full-block-script-tag-cross-domain).

The upside is I can remove a lot of logic that was introduced recently to
preserve pages for view-source of the blocked page.  The window-open-block-mode
test is such an example.  There will be more cleanup possible on the
chrome side once this CL lands.

BUG=396544

Review URL: https://codereview.chromium.org/414223004

git-svn-id: svn://svn.chromium.org/blink/trunk@179240 bbb929c8-8fbe-4397-9dbb-9b2b20218538