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Spec: https://drafts.css-houdini.org/css-typed-om/#computed-stylepropertymap-objects

Uses a  partial IDL interface to attach a getComputedStyleMap method to
window.

Implements the getAll and getProperties.
Moved has() up into the base class from InlineStylePropertyMap.

Not implemented: iteration

BUG=545318

Review-Url: https://codereview.chromium.org/1849653003
Cr-Commit-Position: refs/heads/master@{#397992}