Sync the DOMSettableTokenList interface with the spec
Sync the DOMSettableTokenList interface with the spec

https://dom.spec.whatwg.org/#interface-domsettabletokenlist

BUG=460722

Review URL: https://codereview.chromium.org/955093002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190873 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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