Support for explicit `onclose` handler

[wowczarek]

Some existing browser implementations of HTMLDialogelement already handle the onclose property. Since onclose defines a single handler, it is extremely useful when reusing dialogs, as opposed to addEventListener(), where there is no ready or easy or clean way to clear any existing handlers. With onclose, a dialog element can be used as a template and just have onclose assigned to whatever handler if and when necessary.

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it!) and we'll verify it.

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What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

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[wowczarek]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[googlebot]

CLAs look good, thanks!

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[samthor]

Thanks!