Fix CSP violation report test flakyness.

Some tests, somewhere, set cookies that aren't properly removed, which
is annoying. They're showing up in the CSP violation report tests, as
those currently dump the raw cookie header[1].

This patch adjusts the tests to only write out the single cookie that
the violation report tests care about (and change the name so that it'll
be easy to blame me if this breaks someone else's test).

[1]: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=http%2Ftests%2Fsecurity%2FcontentSecurityPolicy%2Feval-blocked-and-sends-report.html

R=jochen@chromium.org

Review URL: https://chromiumcodereview.appspot.com/18124004

git-svn-id: svn://svn.chromium.org/blink/trunk@154114 bbb929c8-8fbe-4397-9dbb-9b2b20218538