Make the second argument to Element.setAttribute non-optional

This change broke Twitter the first time: http://crbug.com/330261

Twitter was fixed on Dec 20, 2013, so it should be safe to try again.

BUG=325922

Review URL: https://codereview.chromium.org/130193003

git-svn-id: svn://svn.chromium.org/blink/trunk@165028 bbb929c8-8fbe-4397-9dbb-9b2b20218538