Make CompositionEvent.init take a Window rather than WindowProxy

[jamierocks]

This matches all other initUIEvent methods, the existing parameter explanations of the spec, Firefox, and Chromium.

Although WebKit uses WindowProxy, it's worth noting that it uses it in place of Window for other UIEvents too. (they have a FIXME for KeyboardEvent for example)

The following tasks have been completed:

• Confirmed there are no ReSpec/BikeShed errors or warnings.
• Modified Web platform tests (link to pull request)

Implementation commitment:

• WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
• Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=) (N/A as they already use Window)
• Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=) (N/A as they already use Window)