Reset pageVisiblityState in TestRunner::reset.

fast/events/page-visibility-iframe-delete-test.html was not properly
reseting the page visibility at the end of the test. Which meant
that every subsequent test had throttled setTimeouts.

-Simplify the logic of the test. It had a bunch of unnecessary
bools and whatnot.
-Reset pageVisibilityState in TestRunner::reset and remove
the TestRunner API. There should never be a need for a test to
call this method as it's only meant to set the initial visibility.
-Make dispatching the visibilitychange event be the last thing that setVisibilityState does. Otherwise, we fire the event when we've changed the visibility, but before the timer throttling.
-Remove the resetPageVisibility calls from all the tests.

BUG=246107
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/18889003

git-svn-id: svn://svn.chromium.org/blink/trunk@154142 bbb929c8-8fbe-4397-9dbb-9b2b20218538