Add explicit [Exposed] + default dictionary value

[saschanaz]

Required after heycam/webidl#423 and heycam/webidl#750. (Found from web-platform-tests/wpt#18382)

The following tasks have been completed:

• [ ] Modified Web platform tests (link to pull request)

Implementation commitment:

• [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=180469)
• [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=792432 https://bugs.chromium.org/p/chromium/issues/detail?id=948139)
• [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=1423562 https://bugzilla.mozilla.org/show_bug.cgi?id=1562680)

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Preview | Diff

[yoavweiss]

/cc @szager-chromium

[saschanaz]

Fixed by #414