Allow Delete All Cookies to delete *all* browser cookies, regardless of the current domain

[sidonath]

After #1149 was applied it's not clear whether "Delete All Cookies" should delete all cookies for the current domain/origin, or if it should delete all cookies for all domains stored by the browser.

Chrome and Firefox seem to treat this differently: Chrome will delete all cookies, regardless of the current address, but Firefox will delete only those cookies associated with the current domain.

The Firefox behavior causes a race condition when resetting a browser session in tests. In our case, using Capybara (please see the related issue), we delete cookies and then navigate to about:blank. Sometimes, after cookies have been cleared, a long-running ajax request sets a new cookie just before the browser leaves the page for about:blank. Once we return to the same domain we cannot be sure that the browser didn't set any cookies (and that leads to flaky test failures).

It would help to clarify in spec that the browser should delete all cookies, regardless of the current address (i.e. to standardize Chrome behavior). Alternatively, a new API endpoint could be added that explicitly deletes all browser cookies.