Sync the DOMStringMap interface with the spec
Sync the DOMStringMap interface with the spec

https://html.spec.whatwg.org/#the-domstringmap-interface

The spec IDL has [Exposed=(Window,Worker)], but that seems wrong:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=28104

The creator keyword was not added:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=28121

There are no changes to the generated code.

BUG=460722, 462920

Review URL: https://codereview.chromium.org/972493002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@191064 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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