Make Window.alert/confirm/prompt default values the empty string

http://whatwg.org/html#the-window-object

Ad-hoc testing in Firefox Nightly and Opera 12.16 (Presto) reveal that
these browsers already treat missing arguments as the empty string.

Since this is not Web-exposed behavior, the risk is low.

The test case description no longer made sense, so rather than writing
new instructions for manual testing, just let -expected.txt say it.

BUG=335855

Review URL: https://codereview.chromium.org/142263002

git-svn-id: svn://svn.chromium.org/blink/trunk@165443 bbb929c8-8fbe-4397-9dbb-9b2b20218538