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This moves the null-check out into generated bindings code. Since there's
no longer a need to throw (explicitly) the ExceptionState argument can be
dropped from the signatures.

Review URL: https://codereview.chromium.org/232913004

git-svn-id: svn://svn.chromium.org/blink/trunk@171471 bbb929c8-8fbe-4397-9dbb-9b2b20218538