Expand classList test to cover exception in toString

When add and remove is called and there is an exception being thrown
in one of the arguments we need to ensure that we are not calling the
implementation of add and remove.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22880005

git-svn-id: svn://svn.chromium.org/blink/trunk@155973 bbb929c8-8fbe-4397-9dbb-9b2b20218538