Delete cookies in TestRunner::reset.

http/tests/security/aboutBlank/security-context-* are failing due to
http/tests/security/svg-image-leak.html leaking cookies into those tests.
We should clear cookies between tests. Unfortunately, cookies can still
leak if there's a network request from the previous test that finishes
after TestRunner::reset has been called, but that's a separate issue.

BUG=259923

Review URL: https://chromiumcodereview.appspot.com/19081004

git-svn-id: svn://svn.chromium.org/blink/trunk@154378 bbb929c8-8fbe-4397-9dbb-9b2b20218538